Commitdiff and fix for issue #1 improved; switched to GitBlit 1.6.2

* Commitdiff: see
  https://code.google.com/p/gitblit/issues/detail?id=450
  
* Issue #1: in some cases, there were still problems reloading the
  plugin. Now tries better to shutdown properly, and does more on
  re-start to get rid of stale Wicket objects.
  
* GitBlit 1.6.2 includes three bug fixes.