submodule.c: port is_submodule_modified to use porcelain 2

submodule.c: port is_submodule_modified to use porcelain 2

Migrate 'is_submodule_modified' to the new porcelain format of
git-status. This conversion attempts to convert faithfully, i.e.
the behavior ought to be exactly the same.

As the output in the parsing only distinguishes between untracked files
and the rest, this is easy to port to the new format, as we only
need to identify untracked files and the rest is handled in the "else"
case.

untracked files are indicated by only a single question mark instead of
two question marks, so the conversion is easy.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>