revision: insert unsorted, then sort in prepare_revision_walk()

Speed up prepare_revision_walk() by adding commits without sorting
to the commit_list and at the end sort the list in one go.  Thanks
to mergesort() working behind the scenes, this is a lot faster for
large numbers of commits than the current insert sort.

Also introduce and use commit_list_reverse(), to keep the ordering
of commits sharing the same commit date unchanged.  That's because
commit_list_insert_by_date() sorts commits with descending date,
but adds later entries with the same date entries last, while
commit_list_insert() always inserts entries at the top.  The
following commit_list_sort_by_date() keeps the order of entries
sharing the same date.

Jeff's test case, in a repo with lots of refs, was to run:

  # make a new commit on top of HEAD, but not yet referenced
  sha1=`git commit-tree HEAD^{tree} -p HEAD </dev/null`

  # now do the same "connected" test that receive-pack would do
  git rev-list --objects $sha1 --not --all

With a git.git with a ref for each revision, master needs (best of
five):

	real	0m2.210s
	user	0m2.188s
	sys	0m0.016s

And with this patch:

	real	0m0.480s
	user	0m0.456s
	sys	0m0.020s

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -361,6 +361,21 @@ struct commit_list *commit_list_insert(struct commit *item, struct commit_list *
 	return new_list;
 }
 
+void commit_list_reverse(struct commit_list **list_p)
+{
+	struct commit_list *prev = NULL, *curr = *list_p, *next;
+
+	if (!list_p)
+		return;
+	while (curr) {
+		next = curr->next;
+		curr->next = prev;
+		prev = curr;
+		curr = next;
+	}
+	*list_p = prev;
+}
+
 unsigned commit_list_count(const struct commit_list *l)
 {
 	unsigned c = 0;

commit.h

@@ -57,6 +57,7 @@ unsigned commit_list_count(const struct commit_list *l);
 struct commit_list *commit_list_insert_by_date(struct commit *item,
 				    struct commit_list **list);
 void commit_list_sort_by_date(struct commit_list **list);
+void commit_list_reverse(struct commit_list **list_p);
 
 void free_commit_list(struct commit_list *list);
 

revision.c

@@ -2054,11 +2054,13 @@ int prepare_revision_walk(struct rev_info *revs)
 		if (commit) {
 			if (!(commit->object.flags & SEEN)) {
 				commit->object.flags |= SEEN;
-				commit_list_insert_by_date(commit, &revs->commits);
+				commit_list_insert(commit, &revs->commits);
 			}
 		}
 		e++;
 	}
+	commit_list_reverse(&revs->commits);
+	commit_list_sort_by_date(&revs->commits);
 	if (!revs->leak_pending)
 		free(list);