Index format version 4 was introduced in C git in 2012. It's about
time that JGit can deal with it.
Version 4 added prefix path compression. Instead of writing the full
path for each index entry to disk, only the difference to the previous
entry's path is written: a variable-encoded int telling how many bytes
to remove from the previous entry's path to get the common prefix,
followed by the new suffix.
Also, cache entries in a version 4 index are not padded anymore.
Internally, version 3 and version 4 index entries are identical; it's
only the stored format that changes.
Implement this path compression, and make sure we write an index file
that we read previously in the same format. (Only changing from version
2 to version 3 if there are extended flags.)
Add support for the "feature.manyFiles" and the "index.version" git
configs, and honor them when writing a new index file.
Add tests, including a compatibility test that verifies that JGit can
read a version 4 index generated by C git and write an identical
version 4 index.
Bug: 565774
Change-Id: Id83241cf009e50f950eb42f8d56b834fb47da1ed
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
e9cb0a8e47