add get_heads()->consider_depends_on

add get_heads()->consider_depends_on

    Added :paramref:`.ScriptDirectory.get_heads.consider_depends_on`
    parameter to :meth:`.ScriptDirectory.get_heads`. When set to ``True``,
    head revisions that are also a dependency of another revision via
    ``depends_on`` are excluded from the result, matching the effective
    heads that would be present in the ``alembic_version`` table after
    running all upgrades.

Fixes: #1806
Change-Id: I477725580e5ca525c6c1097f95ba0cf705a0244d