DirCache: support option index.skipHash

DirCache: support option index.skipHash

Support the new option index.skipHash which was introduced in git 2.40
[1]. If it is set to true skip computing the git index checksum. This
accelerates Git commands that manipulate the index, such as git add, git
commit, or git status. Instead of storing the checksum, write a trailing
set of bytes with value zero, indicating that the computation was
skipped.

Accept a skipped checksum consisting of 20 null bytes when reading the
index since the option could have been set to true at the time when the
index was written.

[1] https://git-scm.com/docs/git-config#Documentation/git-config.txt-indexskipHash

Bug: 581723
Change-Id: I28ebe44c5ca1cbcb882438665d686452a0c111b2