scripts: Introduce a default git.orderFile

When reviewing patches, it looks much nicer to have some changes shown
before others, which allow better understanding of the patch before the
the .c files reviewing.

Introduce a default git.orderFile, in order to help developers getting the
best ordering easier.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>