range-diff: optionally accept pathspecs

The `git range-diff` command can be quite expensive, which is not a
surprise given that the underlying algorithm to match up pairs of
commits between the provided two commit ranges has a cubic runtime.

Therefore it makes sense to restrict the commit ranges as much as
possible, to reduce the amount of input to that O(N^3) algorithm.

In chatty repositories with wide trees, this is not necessarily
possible merely by choosing commit ranges wisely.

Let's give users another option to restrict the commit ranges: by
providing a pathspec. That helps in repositories with wide trees because
it is likely that the user has a good idea which subset of the tree they
are actually interested in.

Example:

	git range-diff upstream/main upstream/seen HEAD -- range-diff.c

This shows commits that are either in the local branch or in `seen`, but
not in `main`, skipping all commits that do not touch `range-diff.c`.

Note: Since we piggy-back the pathspecs onto the `other_arg` mechanism
that was introduced to be able to pass through the `--notes` option to
the revision machinery, we must now ensure that the `other_arg` array is
appended at the end (the revision range must come before the pathspecs,
if any).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -57,9 +57,9 @@ static int read_patches(const char *range, struct string_list *list,
 		     "--pretty=medium",
 		     "--notes",
 		     NULL);
+	strvec_push(&cp.args, range);
 	if (other_arg)
 		strvec_pushv(&cp.args, other_arg->v);
-	strvec_push(&cp.args, range);
 	cp.out = -1;
 	cp.no_stdin = 1;
 	cp.git_cmd = 1;