stash: add back the original, scripted git stash

This simply copies the version as of sd/stash-wo-user-name verbatim. As
of now, it is not hooked up.

The next commit will change the builtin `stash` to hand off to the
scripted `git stash` when `stash.useBuiltin=false`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>