Git.pm: do not break inheritance

Make it possible to write subclasses of Git.pm
Signed-off-by: NChristian Jaeger <christian@jaeger.mine.nu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

perl/Git.pm

@@ -1203,8 +1203,7 @@ =head1 COPYRIGHT
 # the method was called upon an instance and (undef, @args) if
 # it was called directly.
 sub _maybe_self {
-	# This breaks inheritance. Oh well.
-	ref $_[0] eq 'Git' ? @_ : (undef, @_);
+	UNIVERSAL::isa($_[0], 'Git') ? @_ : (undef, @_);
 }
 
 # Check if the command id is something reasonable.