Create branch at commit

Requested here: https://answers.atlassian.com/questions/54861/how-can-i-create-a-branch-in-sourcetree-git-from-arbitrary-commit

Equivalent to 'git branch <name> <SHA>'.

Still some questions over exact use case, whether we should default to checking out the new branch or whether the reason it would be used was because the user didn't want to check out first.