Merge branch 'sj/use-mmap-to-check-packed-refs'

The code path to access the "packed-refs" file while "fsck" is
taught to mmap the file, instead of reading the whole file in the
memory.

* sj/use-mmap-to-check-packed-refs:
  packed-backend: mmap large "packed-refs" file during fsck
  packed-backend: extract snapshot allocation in `load_contents`
  packed-backend: fsck should warn when "packed-refs" file is empty