merge-ort: add helper functions for using cached renames

merge-ort: add helper functions for using cached renames

If we have a usable rename cache, then we can remove from
relevant_sources all the paths that were cached;
diffcore_rename_extended() can then consider an even smaller set of
relevant_sources in its rename detection.

However, when diffcore_rename_extended() is done, we will need to take
the renames it detected and then add back in all the ones we had cached
from before.

Add helper functions for doing these two operations; the next commit
will make use of them.

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