Ability to checkout a branch with gh/clone

[jsok]

Describe Request:

When specifying a repository to clone, you cannot specify a branch to clone.
Ideally it should checkout $CIRCLE_BRANCH by default.

Examples:

e.g. to clone via https:

      - gh/clone:
          repo: user/repo

Supporting Documentation Links:

It is possible to clone a branch like gh repo clone user/repo -- --branch=my-branch, see also cli/cli#5847