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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2