Update app/controllers/gitolite_hook_controller.rb

Fixed git command to update the mirror (bare) repos correctly, prior to this commit, branches were getting overwritten and therefore not appearing correctly in the Repository Browser.
Update app/controllers/gitolite_hook_controller.rb

Added --prune option to git remote update, so that it picks up deleted branches and other such changes.
Merge pull request #46 from roadhead/patch-1

Fix branches showing up incorrectly in the Repository Browser