Resurrect git-revert.sh example and add comment to builtin-revert.c

I had to scratch my head for quite some time figuring out why we
cannot optimize out write_tree() we do when --no-commit option
is given, whose purpose seem to be only to check if the index is
unmerged, with a simple loop over the active_cache[].

So add a comment to describe why the write_tree() is there, and
resurrect the last scripted version as a reference material in
contrib/example directory with others.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-revert.c

@@ -246,7 +246,9 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	if (no_commit) {
 		/*
 		 * We do not intend to commit immediately.  We just want to
-		 * merge the differences in.
+		 * merge the differences in, so let's compute the tree
+		 * that represents the "current" state for merge-recursive
+		 * to work on.
 		 */
 		if (write_tree(head, 0, NULL))
 			die ("Your index file is unmerged.");