The `Patch` API and semantics are changed a little bit:
* It's now valid again to propose a revision without being the patch
author.
* To get revisions by the patch author, `Patch::updates` is used.
* `Patch::latest` now only looks at revisions by the patch author.
* Added `Patch::revisions_by`, `Patch::authors` and `Patch::latest_by`.
* The merge behavior on git-push is improved to favor revisions that
merge more recent commits. We also allow for merging the non-latest
revision.