update-index: manually enable or disable untracked cache

Some numbers below. In short, the saving on wt_status_collect_untracked()
is about 80%. There are some overhead on read/write_cache, but it seems
lower than 50ms, while the .._untracked() saving is in the 500ms range
(except linux-2.6, about 150ms). "git status" time saving ranges from
33% to 42%.

On gentoo-x86.git (100k files, 23k dirs, quite balance
tree, 8.5MB index v4, cache-tree fully populated), before turning
untracked cache on (the most important line is wt_status_collect:625)

   184.650 gitmodules_config:201 if (read_cache() < 0) die("index
     0.004 cmd_status:1299 read_cache_preload(&s.pathspec)
   226.231 cmd_status:1300 refresh_index(&the_index, REFRESH_QUIET
     3.096 cmd_status:1304 update_index_if_able(&the_index, &index_lock)
     6.788  wt_status_collect:619 wt_status_collect_changes_worktree(s)
     6.780  wt_status_collect:624 wt_status_collect_changes_index(s)
   772.866  wt_status_collect:625 wt_status_collect_untracked(s)
   786.686 cmd_status:1308 wt_status_collect(&s)

real    0m1.211s
user    0m0.566s
sys     0m0.638s

and after (saving 42% total time):

   220.888 gitmodules_config:201 if (read_cache() < 0) die("index
    36.368 cmd_status:1299 read_cache_preload(&s.pathspec)
   223.936 cmd_status:1300 refresh_index(&the_index, REFRESH_QUIET
     2.935 cmd_status:1304 update_index_if_able(&the_index, &index_lock)
     7.031  wt_status_collect:619 wt_status_collect_changes_worktree(s)
     7.156  wt_status_collect:624 wt_status_collect_changes_index(s)
   148.022  wt_status_collect:625 wt_status_collect_untracked(s)
   162.443 cmd_status:1308 wt_status_collect(&s)
    36.943 cmd_status:1348 if (the_index.untracked) { struct

real    0m0.696s
user    0m0.406s
sys     0m0.288s

On webkit.git (182k files, 6k dirs, 14M index v4), before:

   283.182 gitmodules_config:201 if (read_cache() < 0) die("index
     0.004 cmd_status:1299 read_cache_preload(&s.pathspec)
   515.836 cmd_status:1300 refresh_index(&the_index, REFRESH_QUIET
     5.428 cmd_status:1304 update_index_if_able(&the_index, &index_lock)
    11.162  wt_status_collect:619 wt_status_collect_changes_worktree(s)
    11.068  wt_status_collect:624 wt_status_collect_changes_index(s)
   887.247  wt_status_collect:625 wt_status_collect_untracked(s)
   909.722 cmd_status:1308 wt_status_collect(&s)

real    0m1.729s
user    0m0.785s
sys     0m0.941s

and after (saving 38% total time):

   290.994 gitmodules_config:201 if (read_cache() < 0) die("index
    10.132 cmd_status:1299 read_cache_preload(&s.pathspec)
   516.656 cmd_status:1300 refresh_index(&the_index, REFRESH_QUIET
     5.159 cmd_status:1304 update_index_if_able(&the_index, &index_lock)
    12.605  wt_status_collect:619 wt_status_collect_changes_worktree(s)
    11.262  wt_status_collect:624 wt_status_collect_changes_index(s)
   186.032  wt_status_collect:625 wt_status_collect_untracked(s)
   210.134 cmd_status:1308 wt_status_collect(&s)
    12.332 cmd_status:1348 if (the_index.untracked) { struct index_state

real    0m1.058s
user    0m0.525s
sys     0m0.532s

And linux-2.6 (45k files, 3k dirs, 3.2MB index v4), before:

    68.668 gitmodules_config:201 if (read_cache() < 0) die("index
     0.004 cmd_status:1299 read_cache_preload(&s.pathspec)
   114.270 cmd_status:1300 refresh_index(&the_index, REFRESH_QUIET
     1.180 cmd_status:1304 update_index_if_able(&the_index, &index_lock)
     4.027  wt_status_collect:619 wt_status_collect_changes_worktree(s)
     4.265  wt_status_collect:624 wt_status_collect_changes_index(s)
   191.285  wt_status_collect:625 wt_status_collect_untracked(s)
   199.825 cmd_status:1308 wt_status_collect(&s)

real    0m0.392s
user    0m0.177s
sys     0m0.215s

and after (saving 33%):

    71.756 gitmodules_config:201 if (read_cache() < 0) die("index
     5.201 cmd_status:1299 read_cache_preload(&s.pathspec)
   111.064 cmd_status:1300 refresh_index(&the_index, REFRESH_QUIET
     1.171 cmd_status:1304 update_index_if_able(&the_index, &index_lock)
     3.054  wt_status_collect:619 wt_status_collect_changes_worktree(s)
     4.945  wt_status_collect:624 wt_status_collect_changes_index(s)
    27.203  wt_status_collect:625 wt_status_collect_untracked(s)
    35.475 cmd_status:1308 wt_status_collect(&s)
    25.759 cmd_status:1348 if (the_index.untracked) { struct index_state

real    0m0.259s
user    0m0.106s
sys     0m0.132s
test code
TIME() macro for measuring how long a block is executed
time