feat: restart repo renovate after automerge (#751)

Previously, the automerge feature was causing some undesirable behaviour when multiple branches were open at the same time. Example: #707. The main problem is that other branches will still be calculated based on the original `package.json` contents and not the post-merge contents. The simplest solution seems to be:
- Stop all subsequent branch processing after any automerge
- Restart repository renovation
This continues until no branch has automerged in a cycle.

Closes #750