changegroup: pass function to resolve delta parents into constructor
Previously, _deltaparent() encapsulated the logic for all 3
delta parent modes of operation. The choice of delta parent
is static for the lifetime of the packer and can be passed into
the packer as a callable. So do that.
Differential Revision: https://phab.mercurial-scm.org/D4132
$ 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