merge: enable progress reporting for rename detection

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>