odb_pack: don't do ambiguity checks for fully qualified SHA1 hashes

This makes libgit2 more closely match Git, which only checks for
ambiguous pack entries when given short hashes.

Note that the only time this is ever relevant is when a pack has the
same object more than once (it's happened in the wild, I promise).

src/pack.c

@@ -722,7 +722,7 @@ static int pack_entry_find_offset(
 		}
 	}
 
-	if (found && pos + 1 < (int)p->num_objects) {
+	if (found && len != GIT_OID_HEXSZ && pos + 1 < (int)p->num_objects) {
 		/* Check for ambiguousity */
 		const unsigned char *next = current + stride;