Prior to this changeset, rebase always left the working directory as a parent of
the last rebased changeset. The is dubious when, before the rebase, the working
directory was not a parent of the tip most rebased changeset.
With this changeset, we move the working directory back to its original parent.
If the original parent was rebased, we use it's successors.
This is a step toward solving issue3813 (rebase loses active bookmark if it's
not on a head)
The previous error message had two issues: The first issue was that it
wasn't, in fact, an error but a warning, even though it described a
fatal error condition preventing the successful completion of the
command. The second was that it didn't mention the immutable
changesets, leaving the user guessing at the true cause of the error.
The main downside to this change is that we now get an 'abort: can't
abort...' message which technically contradicts itself. In this case,
I blame that on the two uses we have for the word; if it weren't for
backwards compatibility, we could make util.Abort print out 'error:
<whatever>'.
