Make sure get_sha1 does not accept ambiguous sha1 prefix.
Make sure get_sha1 does not accept ambiguous sha1 prefix.

The original code did not even check alternates, and was confused if
an unpacked object was uniquely found when there was another object
that shares the same prefix in the pack.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git