rebase: add --confirm option
This feature adds a functionality in rebase to confirm before applying
When there is no conflict and user confirm to apply actions, we just
finish the unfinished rebase. But when there is a conflict and user
confirm to apply actions then we can't just finish rebasing using
rbsrt._finishrebase() because in-memory merge doesn't support conflicts, so
we have to abort and run on-disk merge in this case.
And if user doesn't confirm to apply actions then simply abort the rebase.
Differential Revision: https://phab.mercurial-scm.org/D3870