.gitignore: ignore *.cover and *.mbx

The 'b4' command creates a *.mbx file, and also a *.cover file if the
patch set has a cover-letter. Ignore them.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>