Merge branch 'ps/midx-negative-packfile-cache'

When a stale .midx file refers to .pack files that no longer exist,
we ended up checking for these non-existent files repeatedly, which
has been optimized by memoizing the non-existence.

* ps/midx-negative-packfile-cache:
  midx: stop repeatedly looking up nonexistent packfiles
  packfile: explain ordering of how we look up auxiliary pack files

diff --cc packfile.c
Simple merge