Allow more changes without changes files.

I suggest that we say that the following kinds of changes do not require a changes file:

Any change to a file that is not distributed in the tarball. This includes:
- Any change to our CI configuration that does not affect the distributed source.
- Any change to developer-only tools, unless those tools are distributed in the tarball.
Non-functional code movement.
Identifier re-namings, comment edits, spelling fixes, and so on.

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