stash: convert create to builtin

Add stash create to the helper.

Signed-off-by: Paul-Sebastian Ungureanu <[email protected]>
Helped-by: Matthew Kraai <[email protected]>
Signed-off-by: Thomas Gummerer <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ungps