rebase: move "backup" flag to rebaseruntime...
rebase: move "backup" flag to rebaseruntime
It was getting messy to populate "backup" flag to calls
of `_finishrebase()` and `_prepareabortorcontinue`, so
made some changes to move "backup" flag to rbsrt.
Differential Revision:
https://phab.mercurial-scm.org/D4055
