update-index: manually enable or disable untracked cache

Overall time saving on "git status" is about 40% in the best case
scenario, removing ..collect_untracked() as the most time consuming
function. read and refresh index operations are now at the top (which
should drop when index-helper and/or watchman support is added). More
numbers and analysis below.

webkit.git
==========

169k files. 6k dirs. Lots of test data (i.e. not touched most of the
time)

Base status
-----------

Index version 4 in split index mode and cache-tree populated. No
untracked cache. It shows how time is consumed by "git status". The
same settings are used for other repos below.

18:28:10.199679 builtin/commit.c:1394   performance: 0.000000451 s: cmd_status:setup
18:28:10.474847 read-cache.c:1407       performance: 0.274873831 s: read_index
18:28:10.475295 read-cache.c:1407       performance: 0.000000656 s: read_index
18:28:10.728443 preload-index.c:131     performance: 0.253147487 s: read_index_preload
18:28:10.741422 read-cache.c:1254       performance: 0.012868340 s: refresh_index
18:28:10.752300 wt-status.c:623         performance: 0.010421357 s: wt_status_collect_changes_worktree
18:28:10.762069 wt-status.c:629         performance: 0.009644748 s: wt_status_collect_changes_index
18:28:11.601019 wt-status.c:632         performance: 0.838859547 s: wt_status_collect_untracked
18:28:11.605939 builtin/commit.c:1421   performance: 0.004835004 s: cmd_status:update_index
18:28:11.606580 trace.c:415             performance: 1.407878388 s: git command: 'git' 'status'

Populating status
-----------------

This is after enabling untracked cache and the cache is still empty.
We see a slight increase in .._collect_untracked() and update_index
(because new cache has to be written to $GIT_DIR/index).

18:28:18.915213 builtin/commit.c:1394   performance: 0.000000326 s: cmd_status:setup
18:28:19.197364 read-cache.c:1407       performance: 0.281901416 s: read_index
18:28:19.197754 read-cache.c:1407       performance: 0.000000546 s: read_index
18:28:19.451355 preload-index.c:131     performance: 0.253599607 s: read_index_preload
18:28:19.464400 read-cache.c:1254       performance: 0.012935336 s: refresh_index
18:28:19.475115 wt-status.c:623         performance: 0.010236920 s: wt_status_collect_changes_worktree
18:28:19.486022 wt-status.c:629         performance: 0.010801685 s: wt_status_collect_changes_index
18:28:20.362660 wt-status.c:632         performance: 0.876551366 s: wt_status_collect_untracked
18:28:20.396199 builtin/commit.c:1421   performance: 0.033447969 s: cmd_status:update_index
18:28:20.396939 trace.c:415             performance: 1.482695902 s: git command: 'git' 'status'

Populated status
----------------

After the cache is populated, wt_status_collect_untracked() drops 82%
from 0.838s to 0.144s. Overall time drops 45%. Top offenders are now
read_index() and read_index_preload().

18:28:20.408605 builtin/commit.c:1394   performance: 0.000000457 s: cmd_status:setup
18:28:20.692864 read-cache.c:1407       performance: 0.283980458 s: read_index
18:28:20.693273 read-cache.c:1407       performance: 0.000000661 s: read_index
18:28:20.958814 preload-index.c:131     performance: 0.265540254 s: read_index_preload
18:28:20.972375 read-cache.c:1254       performance: 0.013437429 s: refresh_index
18:28:20.983959 wt-status.c:623         performance: 0.011146646 s: wt_status_collect_changes_worktree
18:28:20.993948 wt-status.c:629         performance: 0.009879094 s: wt_status_collect_changes_index
18:28:21.138125 wt-status.c:632         performance: 0.144084737 s: wt_status_collect_untracked
18:28:21.173678 builtin/commit.c:1421   performance: 0.035463949 s: cmd_status:update_index
18:28:21.174251 trace.c:415             performance: 0.766707355 s: git command: 'git' 'status'

gentoo-x86.git
==============

This repository is a strange one with a balanced, wide and shallow
worktree (about 100k files and 23k dirs) and no .gitignore in
worktree. .._collect_untracked() time drops 88%, total time drops 56%.

Base status
-----------
18:20:40.828642 builtin/commit.c:1394   performance: 0.000000496 s: cmd_status:setup
18:20:41.027233 read-cache.c:1407       performance: 0.198130532 s: read_index
18:20:41.027670 read-cache.c:1407       performance: 0.000000581 s: read_index
18:20:41.171716 preload-index.c:131     performance: 0.144045594 s: read_index_preload
18:20:41.179171 read-cache.c:1254       performance: 0.007320424 s: refresh_index
18:20:41.185785 wt-status.c:623         performance: 0.006144638 s: wt_status_collect_changes_worktree
18:20:41.192701 wt-status.c:629         performance: 0.006780184 s: wt_status_collect_changes_index
18:20:41.991723 wt-status.c:632         performance: 0.798927029 s: wt_status_collect_untracked
18:20:41.994664 builtin/commit.c:1421   performance: 0.002852772 s: cmd_status:update_index
18:20:41.995458 trace.c:415             performance: 1.168427502 s: git command: 'git' 'status'
Populating status
-----------------
18:20:48.968848 builtin/commit.c:1394   performance: 0.000000380 s: cmd_status:setup
18:20:49.172918 read-cache.c:1407       performance: 0.203734214 s: read_index
18:20:49.173341 read-cache.c:1407       performance: 0.000000562 s: read_index
18:20:49.320013 preload-index.c:131     performance: 0.146671391 s: read_index_preload
18:20:49.328039 read-cache.c:1254       performance: 0.007921957 s: refresh_index
18:20:49.334680 wt-status.c:623         performance: 0.006172020 s: wt_status_collect_changes_worktree
18:20:49.342526 wt-status.c:629         performance: 0.007731746 s: wt_status_collect_changes_index
18:20:50.257510 wt-status.c:632         performance: 0.914864222 s: wt_status_collect_untracked
18:20:50.338371 builtin/commit.c:1421   performance: 0.080776477 s: cmd_status:update_index
18:20:50.338900 trace.c:415             performance: 1.371462446 s: git command: 'git' 'status'
Populated status
----------------
18:20:50.351160 builtin/commit.c:1394   performance: 0.000000571 s: cmd_status:setup
18:20:50.577358 read-cache.c:1407       performance: 0.225917338 s: read_index
18:20:50.577794 read-cache.c:1407       performance: 0.000000617 s: read_index
18:20:50.734140 preload-index.c:131     performance: 0.156345564 s: read_index_preload
18:20:50.745717 read-cache.c:1254       performance: 0.011463075 s: refresh_index
18:20:50.755176 wt-status.c:623         performance: 0.008877929 s: wt_status_collect_changes_worktree
18:20:50.763768 wt-status.c:629         performance: 0.008471633 s: wt_status_collect_changes_index
18:20:50.854885 wt-status.c:632         performance: 0.090988721 s: wt_status_collect_untracked
18:20:50.857765 builtin/commit.c:1421   performance: 0.002789097 s: cmd_status:update_index
18:20:50.858411 trace.c:415             performance: 0.508647673 s: git command: 'git' 'status'

linux-2.6
=========

Reference repo. Not too big. .._collect_status() drops 84%. Total time
drops 42%.

Base status
-----------
18:34:09.870122 builtin/commit.c:1394   performance: 0.000000385 s: cmd_status:setup
18:34:09.943218 read-cache.c:1407       performance: 0.072871177 s: read_index
18:34:09.943614 read-cache.c:1407       performance: 0.000000491 s: read_index
18:34:10.004364 preload-index.c:131     performance: 0.060748102 s: read_index_preload
18:34:10.008190 read-cache.c:1254       performance: 0.003714285 s: refresh_index
18:34:10.012087 wt-status.c:623         performance: 0.002775446 s: wt_status_collect_changes_worktree
18:34:10.016054 wt-status.c:629         performance: 0.003862140 s: wt_status_collect_changes_index
18:34:10.214747 wt-status.c:632         performance: 0.198604837 s: wt_status_collect_untracked
18:34:10.216102 builtin/commit.c:1421   performance: 0.001244166 s: cmd_status:update_index
18:34:10.216817 trace.c:415             performance: 0.347670735 s: git command: 'git' 'status'
Populating status
-----------------
18:34:16.595102 builtin/commit.c:1394   performance: 0.000000456 s: cmd_status:setup
18:34:16.666600 read-cache.c:1407       performance: 0.070992413 s: read_index
18:34:16.667012 read-cache.c:1407       performance: 0.000000606 s: read_index
18:34:16.729375 preload-index.c:131     performance: 0.062362492 s: read_index_preload
18:34:16.732565 read-cache.c:1254       performance: 0.003075517 s: refresh_index
18:34:16.736148 wt-status.c:623         performance: 0.002422201 s: wt_status_collect_changes_worktree
18:34:16.739990 wt-status.c:629         performance: 0.003746618 s: wt_status_collect_changes_index
18:34:16.948505 wt-status.c:632         performance: 0.208426710 s: wt_status_collect_untracked
18:34:16.961744 builtin/commit.c:1421   performance: 0.013151887 s: cmd_status:update_index
18:34:16.962233 trace.c:415             performance: 0.368537535 s: git command: 'git' 'status'
Populated status
----------------
18:34:16.970026 builtin/commit.c:1394   performance: 0.000000631 s: cmd_status:setup
18:34:17.046235 read-cache.c:1407       performance: 0.075904673 s: read_index
18:34:17.046644 read-cache.c:1407       performance: 0.000000681 s: read_index
18:34:17.113564 preload-index.c:131     performance: 0.066920253 s: read_index_preload
18:34:17.117281 read-cache.c:1254       performance: 0.003604055 s: refresh_index
18:34:17.121115 wt-status.c:623         performance: 0.002508345 s: wt_status_collect_changes_worktree
18:34:17.125089 wt-status.c:629         performance: 0.003871636 s: wt_status_collect_changes_index
18:34:17.156089 wt-status.c:632         performance: 0.030895703 s: wt_status_collect_untracked
18:34:17.169861 builtin/commit.c:1421   performance: 0.013686404 s: cmd_status:update_index
18:34:17.170391 trace.c:415             performance: 0.201474531 s: git command: 'git' 'status'
update-index: test the system before enabling untracked cache
t7063: tests for untracked cache
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preload
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