When git-clone creates an initial branch it was not checking the
branch.autosetuprebase configuration option (which may exist in
~/.gitconfig).  Refactor the code used by "git branch" to create
a new branch, and use it instead of the insufficiently duplicated code
in builtin-clone.

Changes are partly, and the test is mostly, based on the previous work by
Pat Notz.

Signed-off-by: Junio C Hamano <gitster@pobox.com>