graft: start storing new nodes formed in graftstate
This patch starts storing the new nodes formed during the ongoing graft
operation in the graftstate. We need the list of new nodes formed while
implmenting `graft --abort` which will strip out the new nodes.
Differential Revision: https://phab.mercurial-scm.org/D3751
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors