diff: fix filtering of merge commits under --remerge-diff

Commit 95433eeed9 ("diff: add ability to insert additional headers for
paths", 2022-02-02) introduced the possibility of additional headers.
Because there could be conflicts with no content differences (e.g. a
modify/delete conflict resolved in favor of taking the modified file
as-is), that commit also modified the diff_queue_is_empty() and
diff_flush_patch() logic to ensure these headers were included even if
there was no associated content diff.

However, the added logic was a bit inconsistent between these two
functions.  diff_queue_is_empty() overlooked the fact that the
additional headers strmap could be non-NULL and empty, which would cause
it to display commits that should have been filtered out.

Fix the diff_queue_is_empty() logic to also account for
additional_path_headers being empty.

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

diff.c

@@ -5925,6 +5925,7 @@ int diff_queue_is_empty(struct diff_options *o)
 	int i;
 	int include_conflict_headers =
 	    (o->additional_path_headers &&
+	     strmap_get_size(o->additional_path_headers) &&
 	     !o->pickaxe_opts &&
 	     (!o->filter || filter_bit_tst(DIFF_STATUS_UNMERGED, o)));
 

t/t4069-remerge-diff.sh

@@ -56,6 +56,11 @@ test_expect_success 'remerge-diff on a clean merge' '
 	test_cmp expect actual
 '
 
+test_expect_success 'remerge-diff on a clean merge with a filter' '
+	git show --oneline --remerge-diff --diff-filter=U bc_resolution >actual &&
+	test_must_be_empty actual
+'
+
 test_expect_success 'remerge-diff with both a resolved conflict and an unrelated change' '
 	git log -1 --oneline ab_resolution >tmp &&
 	cat <<-EOF >>tmp &&