[PATCH] fetch.c: Make process() look at each object only once

The process() function is very often called multiple times for the
same object (because lots of trees refer to the same blobs), but did
not have a fast check for this, therefore a lot of useless calls to
has_sha1_file() and parse_object() were made before discovering that
nothing needs to be done.

This patch adds the SEEN flag which is used in process() to make it
look at each object only once.  When testing git-local-fetch on the
repository of GIT, this gives a 14x improvement in CPU usage (mainly
because the redundant calls to parse_object() are now avoided -
parse_object() always unpacks and parses the object data, even if it
was already parsed before).
Signed-off-by: NSergey Vlasov <vsu@altlinux.ru>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

fetch.c

@@ -58,6 +58,7 @@ static int process_tree(struct tree *tree)
 #define TO_FETCH	2U
 #define TO_SCAN		4U
 #define SCANNED		8U
+#define SEEN		16U
 
 static struct commit_list *complete = NULL;
 
@@ -134,6 +135,10 @@ static int process_object(struct object *obj)
 
 static int process(struct object *obj)
 {
+	if (obj->flags & SEEN)
+		return 0;
+	obj->flags |= SEEN;
+
 	if (has_sha1_file(obj->sha1)) {
 		parse_object(obj->sha1);
 		/* We already have it, so we should scan it now. */