add merge.renamelimit config option

add merge.renamelimit config option

The point of rename limiting is to bound the amount of time
we spend figuring out inexact renames. Currently we use a
single value, diff.renamelimit, for all situations. However,
it is probably the case that a user is willing to spend more
time finding renames during a merge than they are while
looking at git-log.

This patch provides a way of setting those values separately
(though for backwards compatibility, merge still falls back
on the diff renamelimit).

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