git-rev-parse: support show sha1 names for pack entries

This is actually subtly wrong.  If a short match is found in the object
directory, but would _also_ match another SHA1 ID in a pack (or it shows
in one pack but not another), we'll never have done the pack lookup, and
we think it's unique.

I can't find it in myself to care.  You really want to use enough of a
SHA1 that there is never any ambiguity.