Error message from get_sha1() on ambiguous short SHA1.

Unlike cases where "no such object exists", the case where specified
prefix is ambiguous would confuse the user if we say "no such commit"
or such.  Give an extra error message from the uniqueness check if
there are more than one objects that match the given prefix.

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

sha1_name.c

@@ -128,7 +128,7 @@ 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 -1;
+		return error("short SHA1 %.*s is ambiguous.", len, canonical);
 	memcpy(sha1, packed_sha1, 20);
 	return 0;
 }