stash: convert branch to builtin

stash: convert branch to builtin

Add stash branch to the helper and delete the apply_to_branch
function from the shell script.

Checkout does not currently provide a function for checking out
a branch as cmd_checkout does a large amount of sanity checks
first that we require here.

Signed-off-by: Joel Teichroeb <joel@teichroeb.net>
Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>