get_maintainer.pl: fall back to git if only lists are found

It's not 100% obvious to project newcomers that all patches should be sent
there; checkpatch doesn't say so, and since it mentions other lists to CC,
the wording "the list" from the SubmitAPatch wiki page can be taken
to mean only those lists, not the main list too.  We would like therefore
to add a catch-all entry for qemu-devel@nongnu.org.

On its own, this would break fallback to git, because now every file
has a maintainer of sorts.  Modify get_maintainer.pl so that mailing
lists (L: lines) no longer prevent the fallback, only humans (M:
entries).

Several pre-existing entries have a list but no human.  These now
fall back to git.  That's a feature.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: John Snow <jsnow@redhat.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Message-Id: <1454987065-12961-1-git-send-email-swarren@wwwdotorg.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>