detect_tags_changes.py update doesn't work as I'd expect

[Timmmm]

I renamed a tag in this PR: riscv/riscv-isa-manual#2646

If I run make build-tags then it does something very strange - it seems to merge the new name of the tag into ref/riscv-privileged-norm-tags.json, but it still leaves the old one there, and it's still listed in the section tree.

It seems like the code to do this is in this function:

    def update_tags_file(self, file_path: str, changes: TagChanges):
        """Update a tags file by adding new tags from additions.

        Args:
            file_path: Path to the file to update
            changes: Changes detected
        """

I don't understand why we'd ever want to merge stuff like this. All that should happen is it copies the new tags file over the old one. If you want to know what some old tag content was you can just check out an old version of the repo.

[Timmmm]

Also I can't see how to make it actually pass after I've changed a tag. It seems like make check-tags does try to do this merging update thing, but after it's done that it still fails the second time I run it.