shelve: use the class constant in the clear method
This class attribut is used everywhere else. Not using it in clear will get us
in trouble sooner or later.
$ "$TESTDIR/hghave" inotify || exit 80
$ hg init
$ echo "[extensions]" > .hg/hgrc
$ echo "inotify=" >> .hg/hgrc
$ hg inserve -d --pid-file .hg/inotify.pid
$ echo a > a
$ hg ci -Aqm0
$ hg co -q null
$ hg co -q
$ hg st
$ cat a
a
$ "$TESTDIR/killdaemons.py" .hg/inotify.pid