Add exists_prefix to ODB backend and ODB API
Add git_object_short_id API to get short id string

This finds a short id string that will unambiguously select the
given object, starting with the core.abbrev length (usually 7)
and growing until it is no longer ambiguous.
Check short OID len in odb, not in backends
Merge pull request #2159 from libgit2/rb/odb-exists-prefix

Add ODB API to check for existence by prefix and object id shortener