merge_request_diff.rb

Also, improved a bit the method names in
Github::Representation::PullRequest.

Last but not least, ensure temp branch names doesn't contain a `/` as
this would create the ref in a subfolder in `refs/heads` (e.g.
`refs/heads/gh-123/456/rymai/foo`), and would leave empty directories
upon branch deletion (e.g. `refs/heads/gh-123/456/rymai/`.

Signed-off-by: Rémy Coutable <remy@rymai.me>