revision traversal: '--simplify-by-decoration'

With this, you can simplify history not by the contents of the tree, but
whether a commit has been named (ie it's referred to by some branch or
tag) or not.

This makes it possible to see the relationship between different named
commits, without actually seeing any of the details.

When used with pathspec, you would get the usual view that is limited to
the commits that change the contents of the tree plus commits that are
named.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -43,6 +43,7 @@ struct rev_info {
 			lifo:1,
 			topo_order:1,
 			simplify_merges:1,
+			simplify_by_decoration:1,
 			tag_objects:1,
 			tree_objects:1,
 			blob_objects:1,