Create tool to validate that no changes are made to a XEP after the most recent revision block in a commit range

[horazont]

## Summary

In order to keep the attic clean and correct, it is of high importance that the last commit in the main branch which touches a XEP also adds a revision block.

This can be enforced by ensuring that all merges to the main branch also "seal" the changes with a revision block in the last commit which touches a given XEP file.

Work proposal

Use the tool created in #1238 to verify that a tagged XEP file is not changed anymore after the commit which tags it.