feat(switch): change --branch to --create

[mlcui-corp]

git branch uses --branch to create a new branch, but git switch uses --create. This PR changes git-branchless switch to match git switch.

[mlcui-corp]

By the way, feel free to close this if users are used to -b / --branch! git switch has quite a different interface to git checkout, and I feel like it's probably best to match what git switch does instead of git checkout for git-branchless switch.

[arxanas]

@mlcui-google thanks for the fix! Originally, this command was git branchless checkout, so it matched the interface of git checkout. Now it's better to match git switch, like you noticed.

It's slightly unfortunately because git branchless switch also lets you switch to commits, not just branches, so the flag --create becomes ambiguous (is it creating a commit or a branch?). But I doubt anyone will be confused.