stash: optionally use the scripted version again

stash: optionally use the scripted version again

We recently converted the `git stash` command from Unix shell scripts
to builtins.

Let's end users a way out when they discover a bug in the
builtin command: `stash.useBuiltin`.

As the file name `git-stash` is already in use, let's rename the
scripted backend to `git-legacy-stash`.

To make the test suite pass with `stash.useBuiltin=false`, this commit
also backports rudimentary support for `-q` (but only *just* enough
to appease the test suite), and adds a super-ugly hack to force exit
code 129 for `git stash -h`.

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>