FR: Support merge-check delta on PR history GUARDIAN_DIRS

### TL;DR

Current merge-check does not check the history of all PRs and whether entrypoints were force planned/applied via GUARDIAN_DIRS. If a PR in the history (and not in the feature branch history) was force applied this way then we should force the author to rebase too.

### Detailed design

We will need to update [tagrep](https://github.com/abcxyz/tagrep) to fetch the history of all PRs in the git log that are not in the feature branch.

One possible solution is to make tagrep be able to handle an array of target PRs (by commit hash in this case) and introduce a strategy for merging the results from these PRs. In this case we just want a concatenated list of the unique GUARDIAN_DIRS from all PRs in the list.

### Alternatives considered

1. Ignore it. Its a low risk
2. Don't use PR descriptions to trigger plan/applys. Force users to edit files