Recent git versions support worktrees where .git is not a directory but
a file with a path to the .git repository; however the get_maintainer.pl
script only recognises the .git directory, let's fix it.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20191112034532.69079-1-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>