batches: only count changesets that are actually selectable

Created by: courier-new

Another small bug encountered when working through publish from preview.

This PR fixes a small bug that would occur when selecting all visible changesets when some changesets were unselectable because they are hidden external ones. Previously, when ticking the box to select all visible, the small text button with "(Select ##)" would still appear, implying there were more changesets to select, even if all of the remaining changesets were unselectable (see screenshot below). This fix uses the size of pre-filtered changeset IDs, rather than the total number of changesets, to determine how many total can actually be selected.

Before	After