- If you attempted to get a branch feed on a empty repository, it would
result in a panic as the code expects that the branch exists.
- `context.RepoRefByType` would normally already 404 if the branch
doesn't exist, however if a repository is empty, it would not do this
check.
- Fix bug where `/atom/branch/*` would return a RSS feed.
(cherry picked from commit d27bcd98a41b69e313535e5e91e4272136a4bab1)
(cherry picked from commit c58566403df728c1f71b1dd554a573c011a59d7e)
b8b3f6ab8b