fsmonitor: migrate Python ABCs from collections to collections.abc
The Collections Abstract Base Classes in the collections module are
deprecated since Python 3.3 in favor of collections.abc, and removed
in Python 3.10.
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev linkrev nodeid p1-nodeid p2-nodeid
0 0 1e88685f5dde 000000000000 000000000000