git-cvsserver: fix breakage when calling git merge-file

In the same vein as 8336afa,
this fixes the the RCS merge to git-merge-file conversion in
commit e2b7008.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-cvsserver.perl

@@ -946,7 +946,7 @@ sub req_update
 
             $log->debug("Temporary directory for merge is $dir");
 
-            my $return = system("git merge-file", $file_local, $file_old, $file_new);
+            my $return = system("git", "merge-file", $file_local, $file_old, $file_new);
             $return >>= 8;
 
             if ( $return == 0 )