Don't raise RangeNotAncestor for sibling branches

Don't raise RangeNotAncestor for sibling branches

Fixed bug where upgrading to the head of a branch which is already
present would fail, only if that head were also the dependency
of a different branch that is also upgraded, as the revision system
would see this as trying to go in the wrong direction.   The check
here has been refined to distinguish between same-branch revisions
out of order vs. movement along sibling branches.

When we're about to claim an error due to
"alembic upgrade greater to lower", make sure this
isn't a request to hit a node in a different branch
that's already implied.

Change-Id: I8641162bb05c6226f0ea12b88b548df41f5a6b51
Fixes: #336