Add diff command to sync-all

sync-all

@@ -389,6 +389,10 @@ sub scmall {
             scm ($localpath, $scm, "grep", @args)
                 unless $scm eq "darcs";
         }
+        elsif ($command =~ /^diff$/) {
+            scm ($localpath, $scm, "diff", @args)
+                unless $scm eq "darcs";
+        }
         elsif ($command =~ /^clean$/) {
             scm ($localpath, $scm, "clean", @args)
                 unless $scm eq "darcs";
@@ -496,6 +500,7 @@ any extra arguments to git:
   clean
   commit
   config
+  diff
   fetch
   format-patch
   gc