BUG: Github issue/PR preview inconsistency

[bsipocz]

This is an upstream bug, but I wasn't sure which repo it belongs to, so I'll leave the moving up the issue to @choldgraf or another maintainer.

Problem: link to a github PR is resolved as a link to an issue? See the issue icon in the top left corner/title line of the issue in the hover over.

It should be a merge/branch icon instead:

[bsipocz]

Or something like this if we go by how the hover overs look in github itself: