bundle-spec: properly parse boolean configuration as boolean
Before this changesets "v2;revbranchcache=no" would actually request the
addition for a revbranchcache part as the non-empty string `"0"` is `True`
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state