rebase: one safety net, one bugfix and one optimization.

rebase: one safety net, one bugfix and one optimization.

When a .dotest from a previously failed rebase or patch
application exists, rebase got confused and tried to apply
mixture of what was already there and what is being rebased.
Check the existence of the directory and barf.

It failed with an mysterious "fatal: cannot read mbox" message
if the branch being rebased is fully in sync with the base.
Also if the branch is a proper descendant of the base, there is
no need to run rebase logic.  Prevent these from happening by
checking where the merge-base is.

Signed-off-by: Junio C Hamano <junkio@cox.net>