Merge branch 'ps/receive-pack-shallow-optim'

The code to accept shallow "git push" has been optimized.

* ps/receive-pack-shallow-optim:
  commit: use commit graph in `lookup_commit_reference_gently()`
  commit: make `repo_parse_commit_no_graph()` more robust
  commit: avoid parsing non-commits in `lookup_commit_reference_gently()`

diff --cc commit.c
Simple merge

diff --cc commit.h
Simple merge

diff --cc object.h
Simple merge