GitHub PRs use pinned modules from base revision rather than merge target

E.g. this task in the DSI project. It used the pinned version of the genny module rather than the version of master or, what is more correct, the version that was pinned to DSI master.

Rationale: a PR is merging into a moving-target (master). The base commit of a PR is effectively useless for determining correctness of the project and its modules. What we care about is how the project will perform after the merge happens, this requires using the version of modules pinned at master, not the base commit.