rebase: preserve working directory parent (BC)
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)
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3