Merge branch 'dk/format-patch-ignore-diff-submodule' into maint

Setting diff.submodule to 'log' made "git format-patch" produce
broken patches.

* dk/format-patch-ignore-diff-submodule:
format-patch: ignore diff.submodule setting
t4255: test am submodule with diff.submodule

diff --cc builtin/log.c
Simple merge