Allow source links to always point to master

[jrudolph]

That used to be the case, at least accidentally before #198 was fixed. Now the source links point to a branch and you need to change to master manually before you can edit on github.

In the best case, you could choose the branch where the links should go to but it would default to master.

[ennru]

I assume it's the same logic that does the "show snippet source" links which need to point to the correct branch so that the code is the same as shown in the snippet.