Creates a branch of the type that this manager manages and checks it
out.
Parameters:
name – The (short) name of the branch to create.
base – The base commit or ref to base the branch off from. If a base is
not provided explicitly, the default base for this type of branch is
used. See also default_base().
fetch – If set, update the local repo with remote changes prior to
creating the new branch.
must_be_on_default_base – If set, the base must be a valid commit on the branch
manager default_base.