merge-ort: provide helpful submodule update message when possible

merge-ort: provide helpful submodule update message when possible

In commit 4057523a40 ("submodule merge: update conflict error message",
2022-08-04), a more detailed message was provided when submodules
conflict, in order to help users know how to resolve those conflicts.
There were a couple situations for which a different message would be
more appropriate, but that commit left handling those for future work.
Unfortunately, that commit would check if any submodules were of the
type that it didn't know how to explain, and, if so, would avoid
providing the more detailed explanation even for the submodules it did
know how to explain.

Change this to have the code print the helpful messages for the subset
of submodules it knows how to explain.

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