Only block if it was closed unmerged.

Fixes #1205

* check for closed only for monorepos

Renovate already detects PRs mistakenly closed during a certain time period when a bug was present. Now, it will rename those so that they are no longer detected and the ignore code does not have to remain indefinitely.

This feature adds a new behaviour to Renovate, where it will now add a comment to any existing closed PRs that are "blocking" currently valid updates. A new API function "ensureComment" has been added, its purpose is so we can ensure there exists only one comment with a certain subject/heading. This lets us prevent duplicates as well as update existing, without having to keep state about it. ensureComment needs porting to gitlab, but should be quite easy.

replace with findPr

This is a major refactor of branch code to prepare for Yarn workspaces plus creating PRs for branches with failing lockfiles. Marked as "feature" to cause a minor version bump due to the moderate chance of accidentally breaking something.