merge-recursive: make a helper function for cleanup for handle_renames

merge-recursive: make a helper function for cleanup for handle_renames

In anticipation of more involved cleanup to come, make a helper function
for doing the cleanup at the end of handle_renames.  Rename the already
existing cleanup_rename[s]() to final_cleanup_rename[s](), name the new
helper initial_cleanup_rename(), and leave the big comment in the code
about why we can't do all the cleanup at once.

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>