We recently converted the `git stash` command from Unix shell scripts
to builtins.
Just like we have `rebase.useBuiltin` to fall back to the scripted
rebase, to give end users a way out when they discover a bug in the
builtin command, this commit adds support for `stash.useBuiltin`.
This is necessary because Git for Windows wants to ship the builtin
stash earlier than core Git: Git for Windows v2.19.0 will come with
the option of a drastically faster (if a lot less battle-tested)
`git stash`.
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>
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