have index-pack create .keep file more carefully

If by chance we receive a pack which content (list of objects) matches
another pack that we already have, and if that pack is marked with a
.keep file, then we should not overwrite it.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

index-pack.c

@@ -788,14 +788,17 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
 				 get_object_directory(), sha1_to_hex(sha1));
 			keep_name = name;
 		}
-		keep_fd = open(keep_name, O_RDWR | O_CREAT, 0600);
-		if (keep_fd < 0)
-			die("cannot write keep file");
-		if (keep_msg_len > 0) {
-			write_or_die(keep_fd, keep_msg, keep_msg_len);
-			write_or_die(keep_fd, "\n", 1);
+		keep_fd = open(keep_name, O_RDWR|O_CREAT|O_EXCL, 0600);
+		if (keep_fd < 0) {
+			if (errno != EEXIST)
+				die("cannot write keep file");
+		} else {
+			if (keep_msg_len > 0) {
+				write_or_die(keep_fd, keep_msg, keep_msg_len);
+				write_or_die(keep_fd, "\n", 1);
+			}
+			close(keep_fd);
 		}
-		close(keep_fd);
 	}
 
 	if (final_pack_name != curr_pack_name) {