Don't hardcode 'master' into the link generator

[yuvipanda]

Just leaving it empty now checks out HEAD, which is the default branch.

Fixes https://github.com/jupyterhub/nbgitpuller/issues/311

[manics]

It looks like the Binder tab stlil requires a branch, do you consider this within the scope of this PR?

https://nbgitpuller--337.org.readthedocs.build/en/337/link.html

[yuvipanda]

@manics ah damn, it looks like the environment repository branch in the binder generated is actually reused to in the default generator. This sucks, because they both actually need different behavior - mybinder.org understands HEAD, but nbgitpuller does not. Ugh...