templatefilters: raise ProgrammingError if unencodable type passed to json()
This shouldn't happen for any template data types (though I know it does
because of some templater bugs.) Let's clarify it is a bug.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state