branch: move --set-upstream-to behavior to dwim_and_setup_tracking()

branch: move --set-upstream-to behavior to dwim_and_setup_tracking()

This commit is preparation for a future commit that will simplify
create_branch() so that it always creates a branch. This will allow
create_branch() to accept a dry_run parameter (which is needed for "git
branch --recurse-submodules").

create_branch() used to always create a branch, but 4fc5006676 (Add
branch --set-upstream, 2010-01-18) changed it to also be able to set
tracking information without creating a branch.

Refactor the code that sets tracking information into its own functions
dwim_branch_start() and dwim_and_setup_tracking(). Also change an
invocation of create_branch() in cmd_branch() in builtin/branch.c to use
dwim_and_setup_tracking(), since that invocation is only for setting
tracking information (in "git branch --set-upstream-to").

As of this commit, create_branch() is no longer invoked in a way that
does not create branches.

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