rebase: remove unnecessary confirm block
Removed unnecessary 'if confirm:' block as that thing is now
handled at 'finally'.
Differential Revision: https://phab.mercurial-scm.org/D3945
rebase: in --confirm option just abort if hit a conflict
Before this patch, it was prompting the user in both cases 1) when
there is no conflict 2) when there is at least one conflict. But
for simplicity we can just abort if we hit a conflict and no need to
prompt in that case.
Differential Revision: https://phab.mercurial-scm.org/D3944