teach log --no-walk=unsorted, which avoids sorting

When 'git log' is passed the --no-walk option, no revision walk takes
place, naturally. Perhaps somewhat surprisingly, however, the provided
revisions still get sorted by commit date. So e.g 'git log --no-walk
HEAD HEAD~1' and 'git log --no-walk HEAD~1 HEAD' give the same result
(unless the two revisions share the commit date, in which case they
will retain the order given on the command line). As the commit that
introduced --no-walk (8e64006 (Teach revision machinery about
--no-walk, 2007-07-24)) points out, the sorting is intentional, to
allow things like

 git log --abbrev-commit --pretty=oneline --decorate --all --no-walk

to show all refs in order by commit date.

But there are also other cases where the sorting is not wanted, such
as

 <command producing revisions in order> |
       git log --oneline --no-walk --stdin

To accomodate both cases, leave the decision of whether or not to sort
up to the caller, by allowing --no-walk={sorted,unsorted}, defaulting
to 'sorted' for backward-compatibility reasons.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -41,6 +41,10 @@ struct rev_cmdline_info {
 	} *rev;
 };
 
+#define REVISION_WALK_WALK 0
+#define REVISION_WALK_NO_WALK_SORTED 1
+#define REVISION_WALK_NO_WALK_UNSORTED 2
+
 struct rev_info {
 	/* Starting list */
 	struct commit_list *commits;
@@ -62,7 +66,7 @@ struct rev_info {
 	/* Traversal flags */
 	unsigned int	dense:1,
 			prune:1,
-			no_walk:1,
+			no_walk:2,
 			show_all:1,
 			remove_empty_trees:1,
 			simplify_history:1,