merge: enable progress reporting for rename detection

The user can enable or disable it explicitly with the new
--progress, but it defaults to checking isatty(2).

This works only with merge-recursive and subtree. In theory
we could pass a progress flag to other strategies, but none
of them support progress at this point, so let's wait until
they grow such a feature before worrying about propagating
it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.h

@@ -21,6 +21,7 @@ struct merge_options {
 	int merge_rename_limit;
 	int rename_score;
 	int needed_rename_limit;
+	int show_rename_progress;
 	int call_depth;
 	struct strbuf obuf;
 	struct string_list current_file_set;