persistent-nodemap: add a "abort" option to the slow-path config
We make it the default, and document the behavior in the help for the main
config option.
Differential Revision: https://phab.mercurial-scm.org/D9762
$ 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