submodule support

- detect .gitmodules file for submodules, add a link "submodules" to the top menu.
- link each submodule to the .gitmodules file in HEAD.
- revert the special style for modules, but list it as a file "m---------", don't show
  the line/filesize though, it is not a file.

style.css

@@ -58,10 +58,6 @@ table td {
 	white-space: normal;
 }
 
-a.module {
-	color: #000;
-}
-
 td.num {
 	text-align: right;
 }