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