expand_sha(
git_dir,prefix) ->sha,err
Expand an abbreviated object id to its full 40-hex form
Resolves a short object id the way git does: loose objects and every pack
index are searched for ids starting with prefix. Returns nil and
"ambiguous object id" when more than one object matches, so a caller
never acts on a guess. A full 40-character id is returned unchanged
without touching the object database.