Merge branch 'tb/fix-midx-rename-while-mapped'

The codepath to write a new version of .midx multi-pack index files
has learned to release the mmaped memory holding the current
version of .midx before removing them from the disk, as some
platforms do not allow removal of a file that still has mapping.

* tb/fix-midx-rename-while-mapped:
  midx.c: guard against commit_lock_file() failures
  midx.c: lookup MIDX by object directory during repack
  midx.c: lookup MIDX by object directory during expire
  midx.c: extract MIDX lookup by object_dir