From: Philippe Blain <levraiphilippeblain@gmail.com>

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.

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

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1661.git.git.1706534968589.gitgitgadget@gmail.com