Say what the age hook does with a read-only file

The header said a file the pusher cannot write stays as it was, but the
hook replaces it through mv, which needs only the directory, and with
stdin not a terminal mv does so without asking. That is the useful
behaviour, so the header now says so, and a test holds the hook to it.
Only a directory the pusher cannot write keeps the old file, which was
the one case tested.

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