graft: make --force apply across continues (issue3220)
Since --force determines the list of revisions to be grafted, it doesn't really
make sense for users to have to keep typing --force --continue as they continue
grafting.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
revision 0
branch default
phase draft
adda
$ cd ..