revlog-sparse-read: add a lower-threshold for read block size
The option experimental.sparse-read.min-block-size specifies the minimal size
of a deltachain span, under which it won't be split by _slicechunk.
$ 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