Use symbolic name SHORT_NAME_AMBIGUOUS as error return value

Signed-off-by: Uwe Zeisberger <zeisberg@informatik.uni-freiburg.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

sha1_name.c

@@ -143,7 +143,7 @@ static int find_unique_short_object(int len, char *canonical,
 	}
 	/* Both have unique ones -- do they match? */
 	if (memcmp(packed_sha1, unpacked_sha1, 20))
-		return -2;
+		return SHORT_NAME_AMBIGUOUS;
 	memcpy(sha1, packed_sha1, 20);
 	return 0;
 }