diff: fix filtering of additional headers under --remerge-diff

diff: fix filtering of additional headers 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, when the pickaxe is active, we really only want the remerge
conflict headers to be shown when there is an associated content diff.
Adjust the logic in these two functions accordingly.

This also removes the TEST_PASSES_SANITIZE_LEAK=true declaration from
t4069, as there is apparently some kind of memory leak with the pickaxe
code.

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