Do true default remote branch detection

[alerque]

We took one pass at this issue in #294, closed by PR #295, but the solution was to just iteratively check a few possibilities. This is more robust and definitely deserved to be included in the code sooner than later, but I can't help but feeling it's not the best permanent solution.

I'm opening this to track an ultimate solution. As I commented here one potential solution might be to clone first and ask questions later: specifically doing a bare clone and just letting Git find the default branch itself, then inspecting what it came up with and using that info to do a checkout.

[RichiH]

Agreed, doing it fully dynamically would be best. As 80/20 implementation it should get us rather far as master and main most likely comprise near-100% of use cases (unless someone has floating branches rebasing on others for local config or so. But that would most likely mean clean/smudge filters or special server-side code and thus be unlikely)