Log repo pack / ref statistics after repack

We have poor visibility into the shape and size of Git repositories we host. I suggest we start logging small reports every time we do a repack (GarbageCollect, RepackFull, RepackIncremental, FetchIntoObjectPool).

Object statistics:

• number of loose objects
• total size of loose objects
• number of packfiles
• total size of packfiles (just the .pack)
• same statistics for alternates (pool), if present

These can all be obtained from a single walk of objects/.

Ref statistics:

• number of loose refs
• number of top-level namespaces (refs/heads, refs/tags, etc.)
• ref count per namespace for top 5 (e.g. refs/heads 100, refs/keeparound 80, refs/tags 15)

These numbers can be obtained by parsing git for-each-ref and a walk of refs/ (for the loose ref count).