Add back the original, scripted git stash

This simply copies the version as of v2.19.0-rc0 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 <[email protected]>

Author: dscho