gitolite_hooks_controller: fix undefined local variable

I had a Problem with the post-receive-urls, this commit fixed it.

Errormessage:

     NameError (undefined local variable or method `revisions' for #<GitoliteHooksController:0x0000000944eef0>):
      plugins/redmine_git_hosting/app/controllers/gitolite_hooks_controller.rb:292:in `block in post_receive_payloads'
      plugins/redmine_git_hosting/app/controllers/gitolite_hooks_controller.rb:270:in `each'
      plugins/redmine_git_hosting/app/controllers/gitolite_hooks_controller.rb:270:in `post_receive_payloads'
      plugins/redmine_git_hosting/app/controllers/gitolite_hooks_controller.rb:52:in `block in post_receive'
      actionpack (3.2.13) lib/action_dispatch/http/response.rb:44:in `each'