merge-recursive: make !o->detect_rename codepath more obvious

merge-recursive: make !o->detect_rename codepath more obvious

Previously, if !o->detect_rename then get_renames() would return an
empty string_list, and then process_renames() would have nothing to
iterate over.  It seems more straightforward to simply avoid calling
either function in that case.

Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>