stash: convert create to builtin

Add stash create to the helper.

Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Helped-by: Matthew Kraai <mkraai@its.jnj.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ungps