merge-ort: turn submodule conflict suggestions into an advice

merge-ort: turn submodule conflict suggestions into an advice

Add a new advice type 'submoduleMergeConflict' for the error message
shown when a non-trivial submodule conflict is encountered, which
was added in 4057523a40 (submodule merge: update conflict error
message, 2022-08-04). That commit mentions making this message an
advice as possible future work.  The message can now be disabled
with the advice mechanism.

Update the tests as the expected message now appears on stderr instead
of stdout.

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