add status.showUntrackedFiles=all to system config

Desktop received a report about our use of `git status` interfering with a user's preferred config: https://github.com/desktop/desktop/issues/3734

I looked at the behaviour of `git status` and it doesn't seem to support a way for Desktop to receive all untracked file paths (the current behaviour) or hide untracked files (if `status.showUntrackedFiles=no` is set). The defaults favour the `normal` mode which is great for the CLI experience, but not great for apps.

We already have the ability to ship system configuration changes [like this](https://github.com/desktop/dugite-native/blob/master/script/build-win32.sh#L53-L56) and I think, from an application perspective, setting `status.showUntrackedFiles=all` is a reasonable default behaviour - applications don't really get much use out of the "normal" view if they want to look at untracked files.