1. We have a `$srcdir/writable/page.mdwn` source file in Git.
2. ikiwiki is configured to allow edits via the CGI in `writable/*`,
   but nowhere else.
2. Modify `$srcdir/writable/page.mdwn`, commit ⇒ commit `$id`.
3. `git mv $srcdir/writable/page.mdwn $srcdir/read-only/page.mdwn`

⇒ The web interface allows reverting commit `$id` (presumably because
it changes files only in `$srcdir/writable`). This operation
effectively modifies `$srcdir/read-only/page.mdwn`, which feels wrong.
My guess is that `check_canchange` does not take into account that Git
will automatically detect that the file affected by the to-be-reverted
commit has moved, and modify the file in its new location
when reverting.