Allow multiple branches

[AMDmi3]

Consider the following upstream versions:
2.0.0, 1.2.0, 1.1.1, 1.1.0, 1.0.1, 1.0.0

Consider the following repository versions:
2.0.0, 1.1.1. 1.0.0

With setbranchcomps: 1, 1.1.1 is outdated (since there's newer 1.2.0 in the 1.x branch), but 1.0.0 is not.
With setbranchcomps: 2, 1.0.0 is outdated (since there's newer 1.0.1 in 1.0.x branch), but 1.1.1 is not.

However, we probably want to mark both versions outdated, so setbranchcomps: 2 should generate two branches (and there should be support for multiple branches as well).