Propose for merging link is not shown in limited circumstances

If we have a product that's linked to a source package and the product has exactly one branch and the source package has exactly one branch, then we hide the "Propose for merging" link on both branch pages. This is incorrect, since we actually allow merging between linked products and source packages.

This bug is caused by the code that hides the link if it sees only one branch in a branch target (i.e. a product or a source package). One way of addressing this bug is to create a "merge proposal space" abstraction that covers not just a single product or package, but all targets for which branches can be merged into one another.