merge: add merge.renames config setting

merge: add merge.renames config setting

Add the ability to control rename detection for merge via a config setting.
This setting behaves the same and defaults to the value of diff.renames but only
applies to merge.

Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Ben Peart <benpeart@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>