commit-graph: fix documentation inconsistencies

commit-graph: fix documentation inconsistencies

The commit-graph feature shipped in Git 2.18 has some inconsistencies in
the constants used by the implementation and specified by the format
document.

The commit data chunk uses the key "CDAT" in the file format, but was
previously documented to say "CGET".

The commit data chunk stores commit parents using two 32-bit fields that
typically store the integer position of the parent in the list of commit
ids within the commit-graph file. When a parent does not exist, we had
documented the value 0xffffffff, but implemented the value 0x70000000.
This swap is easy to correct in the documentation, but unfortunately
reduces the number of commits that we can store in the commit-graph.
Update that estimate, too.

Reported-by: Grant Welch <gwelch925@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>