Merge branch 'da/difftool-updates' into next

(Originally merged to 'next' on 2012-08-10)

"git difftool --dir-diff" learned to use symbolic links to prepare
temporary copy of the working tree when available.

Will merge to 'master' as part of the second batch.

* da/difftool-updates:
  difftool: silence warning

git-difftool.perl

@@ -138,7 +138,7 @@ sub setup_dir_diff
 
 	my @gitargs = ('diff', '--raw', '--no-abbrev', '-z', @ARGV);
 	my $diffrtn = $diffrepo->command_oneline(@gitargs);
-	exit(0) if (length($diffrtn) == 0);
+	exit(0) unless defined($diffrtn);
 
 	# Build index info for left and right sides of the diff
 	my $submodule_mode = '160000';