The offered default *is* the current branch, therefore there already is a key binding for creating a new branch starting at the current branch. But that binding is <kbd>b c RET</kbd>, not <kbd>b c</kbd>.

Okay that was a slight simplification. If and only if there is a branch at point (e.g. in a log or the references buffer) then that branch is offered as the default instead.

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By the way some changes are planned to how branching and pushing work. But that will take some time.