merge-recursive: Move handling of double rename of one file to other file

merge-recursive: Move handling of double rename of one file to other file

Move the handling of rename/rename conflicts where one file is renamed on
both sides to the same file, from process_renames() to process_entry().
Here we avoid the three way merge logic by just using
update_stages_and_entry() to move the higher stage entries in the index
from the rename source to the rename destination, and then allow
process_entry() to do its magic.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>