t4255: test am submodule with diff.submodule

t4255: test am submodule with diff.submodule

git am will break when using diff.submodule=log; add some test cases
to illustrate this breakage as simply as possible.  There are
currently two ways this can fail:

* With errors ("unrecognized input"), if only change
* Silently (no submodule change), if other files change

Test for both conditions and ensure without diff.submodule this works.

Helped-by: Eric Sunshine <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: Doug Kelly <redacted>
Signed-off-by: Junio C Hamano <redacted>