Turns out there's a much better API for this that doesn't have the
footguns of the previous method.
isLegalRefName is somewhat of a misnomer, since it's mainly used to
validate user inputs that can be either references, branch names,
psedorefs or tags.
Fixes usage of `#` symbol in the reference name.
This also seems to identify several deficiencies in the libgit2 refname
validation code wrt to DEL symbol and a singular `@` symbol [1].
[1]: https://git-scm.com/docs/git-check-ref-format#_description
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/
```
