merge-ort: use relevant_sources to filter possible rename sources

The past several commits determined conditions when rename sources might
be needed, and filled a relevant_sources strset with those paths.  Pass
these along to diffcore_rename_extended() to use to limit the sources
that we need to detect renames for.

For the testcases mentioned in commit 557ac03 ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:       12.596 s ±  0.061 s     6.003 s ±  0.048 s
    mega-renames:    130.465 s ±  0.259 s   114.009 s ±  0.236 s
    just-one-mega:     3.958 s ±  0.010 s     3.489 s ±  0.017 s

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

merge-ort.c

@@ -2208,7 +2208,7 @@ static void detect_regular_renames(struct merge_options *opt,
 	diff_queued_diff = renames->pairs[side_index];
 	trace2_region_enter("diff", "diffcore_rename", opt->repo);
 	diffcore_rename_extended(&diff_opts,
-				 NULL,
+				 &renames->relevant_sources[side_index],
 				 &renames->dirs_removed[side_index],
 				 &renames->dir_rename_count[side_index]);
 	trace2_region_leave("diff", "diffcore_rename", opt->repo);