Impossible to show a blob named `diff` [failure unrelated]

Created by: cirosantilli

Before this commit, it was impossible to see a blob whose filename ends in diff because GET path/to/diff routed to blob#diff.

Currently live on GitLab: https://gitlab.com/cirosantilli/test/blob/master/diff

You will get redirected to https://gitlab.com/cirosantilli/test/tree/master/ where you can see the the file named diff does in fact exist!

This is an ugly fix that replaces the GET with a POST to resolve the ambiguity but for something that does not change system state, but it is the simplest DRY strategy I could find, and the same strategy is currently used for blob edit preview at: https://github.com/gitlabhq/gitlabhq/blob/30c447ed2f35a0d1f4eb0f6200aa9ab952c46768/config/routes.rb#L201

To solve the problem beautifully, we must instead use a route like diff_blob/:id, and to do that nicely someone must answer: http://stackoverflow.com/questions/26223317/how-to-put-a-member-of-a-routing-resource-outside-of-its-parent-namespace