get_maintainer.pl: restrict cases where it falls back to --git

get_maintainer.pl: restrict cases where it falls back to --git

The list emitted by --git-fallback often leads inexperienced contributors
to add pointless CCs.  While not discouraging usage of --git-fallback,
we want to:

1) disable the fallback if only some files lack a maintainer

    $ scripts/get_maintainer.pl -f util/cutils.c hw/ide/core.c
    Kevin Wolf <kwolf@redhat.com> (odd fixer:IDE)
    Stefan Hajnoczi <stefanha@redhat.com> (odd fixer:IDE)

This behavior is taken even if --git-fallback is specified.

2) warn the contributors about what we're doing, asking them to use their
common sense:

    $ scripts/get_maintainer.pl -f util/cutils.c
    get_maintainer.pl: No maintainers found, printing recent contributors.
    get_maintainer.pl: Do not blindly cc: them on patches!  Use common sense.

    Luiz Capitulino <lcapitulino@redhat.com> (commit_signer:1/2=50%)
    ...
    $

Explicitly disabling the fallback will not result in the warning message:

    $ scripts/get_maintainer.pl -f util/cutils.c   --no-git-fallback
    $ echo $?
    0

(Returning 1 would break usage of scripts/get_maintainer.pl as a cccmd
for git-send-email).

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>