branch: add a dry_run parameter to create_branch()

branch: add a dry_run parameter to create_branch()

Add a dry_run parameter to create_branch() such that dry_run = 1 will
validate a new branch without trying to create it. This will be used in
`git branch --recurse-submodules` to ensure that the new branch can be
created in all submodules.

Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>