git-remote-mediawiki: rewrite unclear line of instructions

Subroutines' parameters should be assigned to variable before doing anything
else
Besides, existing instruction affected a variable inside a "if", which break
Git's coding style

Signed-off-by: Célestin Matte <celestin.matte@ensimag.fr>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/mw-to-git/git-remote-mediawiki.perl

@@ -1333,7 +1333,8 @@ sub get_mw_namespace_id {
 }
 
 sub get_mw_namespace_id_for_page {
-	if (my ($namespace) = $_[0] =~ /^([^:]*):/) {
+	my $namespace = shift;
+	if ($namespace =~ /^([^:]*):/) {
 		return get_mw_namespace_id($namespace);
 	} else {
 		return;