- Fixed bug where the case of multiple mergepoints that all

- Fixed bug where the case of multiple mergepoints that all
have the identical set of ancestor revisions would fail to be
upgradable, producing an assertion failure.   Merge points were
previously assumed to always require at least an UPDATE in
alembic_revision from one of the previous revs to the new one,
however in this case, if one of the mergepoints has already
been reached, the remaining mergepoints have no row to UPDATE therefore
they must do an INSERT of their target version.
fixes #297