Move index v2 specific functions to their own file

Move index version 2 specific functions to their own file,
to prepare for the addition of a new index file format. With
the split into two files we have the non-index specific
functions in read-cache.c and the index-v2 specific functions
in read-cache-v2.c

Helped-by: Nguyen Thai Ngoc Duy <pclouds@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
t2104: Don't fail for index versions other than [23]

t2104 currently checks for the exact index version 2 or 3,
depending if there is a skip-worktree flag or not. Other
index versions do not use extended flags and thus cannot
be tested for version changes.

Make this test update the index to version 2 at the beginning
of the test. Testing the skip-worktree flags for the default
index format is still covered by t7011 and t7012.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
t3700: Avoid interfering with the racy code

The new git racy code uses the mtime of cache-entries as smudge
marker for racily clean entries. The work of checking the file-system
if the entry really changed is offloaded to the reader. This interferes
with this test, because the entry is racily smudged and thus has
mtime 0.

To avoid interfering with the racy code, we use a time relative
to the time returned by time(3), instead of a time relative to
the mtime of the cache entries.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Add documentation of the index-v5 file format

Add a documentation of the index file format version 5 to
Documentation/technical.

Helped-by: Michael Haggerty <mhagger@alum.mit.edu>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Thomas Rast <trast@student.ethz.ch>
Helped-by: Nguyen Thai Ngoc Duy <pclouds@gmail.com>
Helped-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Make in-memory format aware of stat_crc

Make the in-memory format aware of the stat_crc used by index-v5.
It is simply ignored by index version prior to v5.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Read index-v5

Make git read the index file version 5 without complaining.

This version of the reader doesn't read neither the cache-tree
nor the resolve undo data, but doesn't choke on an index that
includes such data.

Helped-by: Nguyen Thai Ngoc Duy <pclouds@gmail.com>
Helped-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Read resolve-undo data

Make git read the resolve-undo data from the index.

Since the resolve-undo data is joined with the conflicts in
the ondisk format of the index file version 5, conflicts and
resolved data is read at the same time, and the resolve-undo
data is then converted to the in-memory format.

Helped-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Read cache-tree in index-v5

Since the cache-tree data is saved as part of the directory data,
we already read it at the beginning of the index. The cache-tree
is only converted from this directory data.

The cache-tree data is arranged in a tree, with the children sorted by
pathlen at each node, while the ondisk format is sorted lexically.
So we have to rebuild this format from the on-disk directory list.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Write index-v5

Write the index version 5 file format to disk. This version doesn't
write the cache-tree data and resolve-undo data to the file.

The main work is done when filtering out the directories from the
current in-memory format, where in the same turn also the conflicts
and the file data is calculated.

Helped-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Write index-v5 cache-tree data

Write the cache-tree data for the index version 5 file format. The
in-memory cache-tree data is converted to the ondisk format, by adding
it to the directory entries, that were compiled from the cache-entries
in the step before.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Write resolve-undo data for index-v5

Write the resolve undo data to the ondisk format, by joining the data
in the resolve-undo string-list with the already existing conflicts
that were compiled before, when searching the directories and add
them to the corresponding directory entries.

Helped-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
update-index.c: add a force-rewrite option

Add a force-rewrite option to update-index, which allows the user
to rewrite the index, even if there are no changes. This can be used
to do performance tests of both the reader and the writer.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
p0002-index.sh: add perf test for the index formats

Add a performance test for index version [23]/4/5 by using
git update-index --force-rewrite, thus testing both the reader
and the writer speed of all index formats.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>