Diff and patch for commits and merge requests

Created by: riyad

• Fixes commit patches getting escaped (see #2036 (closed))
• Support diff and patch generation for commits and merge request
• MergeReqest doesn't generate a temporary file for the patch any more
• Update the UI to allow downloading both

Problems with this PR:

• I had to add a constraint to the id in the merge request routes. Otherwise the id was parsed something like "1.patch"
• the merge requests controller spec won't execute the respond_to block. Adding render_views to the spec produces 404 errors. (that's also why the tests fail)

Can anyone see why? What is different in the MergeRequestsController? If noone knows I'll remove the merge request controller spec before merge.