rebase: add a stateobj variable to rebaseruntime class
The stateobj variable is an object of state.cmdstate() and will be used in
upcoming patches to clean up the logic a bit and also use cbor format to write
data to rebasestate class.
Differential Revision: https://phab.mercurial-scm.org/D3874
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors