Serve pull request .diff files (#3293)

* Serve pull request .diff files

Closes #3259

* Add test for pull request redirection and .diff access

* Typo

* There's no need to test for pr.BaseRepo being nil after calling GetBaseRepo

models/repo.go

@@ -570,7 +570,9 @@ func (repo *Repository) GetMirror() (err error) {
 	return err
 }
 
-// GetBaseRepo returns the base repository
+// GetBaseRepo populates repo.BaseRepo for a fork repository and
+// returns an error on failure (NOTE: no error is returned for
+// non-fork repositories, and BaseRepo will be left untouched)
 func (repo *Repository) GetBaseRepo() (err error) {
 	if !repo.IsFork {
 		return nil