Let hub fork copy only the main branch

[joeytwiddle]

The problem I'm trying to solve:

When running hub fork all the branches are copied over to my fork.

This results in a lot of duplicate branches when doing git branch -r

Suggestion: Give hub fork an option (like GitHub website now has) to only copy over the default branch. (This is commonly main or master.)

We could also consider whether it's desirable to make this the default behaviour. (I would be in favour, but it would change hub's traditional behaviour.)

How I imagine hub could expose this functionality:

hub fork --lite

hub fork -l

hub fork --default-only (this is a bit cumbersome)