Stop the merge process once the dependency chain for a PR is modified

[joao-paulo-parity]

Problem: Suppose the following scenario

1. I create a PR with two companions

polkadot companion: [link]
cumulus companion: [link]

2. I say bot merge

3. After bot merge has started, I edit the description of the PR to remove the cumulus companion

polkadot companion: [link]
-cumulus companion: [link]

Now there's a problem that the PR description is no longer matching the dependency chain computed at the start of bot merge.

Solution: stop the merge process once the dependency chain for a PR is modified. When that happens, post a comment "Error: PR dependency chain was modified" with a diff between old/new dependencies.