[bug] Incorrect URL in post-receive JSON data for commits

Created by: rlaneve

Within roles/push_observer.rb, the url generated for each commit being included in the post_receive_data uses:

"#{Gitlab.config.url}/#{path}/commits/#{commit.id}"

Such a URL always returns the full list of commits for the related repo.

To display the specific commit as intended, it should actually be the singular form of the word "commit", like this:

"#{Gitlab.config.url}/#{path}/commit/#{commit.id}"

Relevant file+line here: https://github.com/gitlabhq/gitlabhq/blob/master/app/roles/push_observer.rb#L117