Don't attempt to `git add` ignored files
(cherry picked from commit 359990dfdc713c80aabd7ea6f7e4528628fbe108)
===
also added a regression test that isn't upstream to be sure we're
actually fixing the bug.
Change-Id: I8267a3d0ece9909d8008b7435b90e7b3eee366f6
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)
eldritch horrors
