checkout: no progress messages if !isatty(2).

If stderr isn't a tty, we shouldn't be printing incremental progress
messages.  In particular, this affects 'git checkout -f . >&logfile'
unless you provided -q.  And git-new-workdir has no way to provide -q.

It would probably be better to have progress.c check isatty(2) all the time,
but that wouldn't allow things like 'git push --progress' to force progress
reporting to on, so I won't try to solve the general case right now.

Actual fix suggested by Jeff King.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -343,7 +343,7 @@ static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
 	opts.reset = 1;
 	opts.merge = 1;
 	opts.fn = oneway_merge;
-	opts.verbose_update = !o->quiet;
+	opts.verbose_update = !o->quiet && isatty(2);
 	opts.src_index = &the_index;
 	opts.dst_index = &the_index;
 	parse_tree(tree);
@@ -420,7 +420,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 		topts.update = 1;
 		topts.merge = 1;
 		topts.gently = opts->merge && old->commit;
-		topts.verbose_update = !opts->quiet;
+		topts.verbose_update = !opts->quiet && isatty(2);
 		topts.fn = twoway_merge;
 		if (opts->overwrite_ignore) {
 			topts.dir = xcalloc(1, sizeof(*topts.dir));