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 <sunshine@sunshineco.com> Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Doug Kelly <dougk.ff7@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>