Merge branch 'rs/commit-stack'

Code clean-up, unifying various hand-rolled "list of commit
objects" and use the commit_stack API.

* rs/commit-stack:
  commit-reach: use commit_stack
  commit-graph: use commit_stack
  commit: add commit_stack_grow()
  shallow: use commit_stack
  pack-bitmap-write: use commit_stack
  commit: add commit_stack_init()
  test-reach: use commit_stack
  remote: use commit_stack for src_commits
  remote: use commit_stack for sent_tips
  remote: use commit_stack for local_commits
  name-rev: use commit_stack
  midx: use commit_stack
  log: use commit_stack
  revision: export commit_stack

diff --cc commit.c
Simple merge

diff --cc midx-write.c
Simple merge