Enabled for fetchGit, which historically had this behavior,
among other behaviors we do not want in fetchGit.
fetchTree disables this parameter by default. It can choose the
simpler behavior, as it is still experimental.
I am not confident that the filtering implementation is future
proof. It should reuse a source filtering wrapper, which I believe
Eelco has already written, but not merged yet.
Two changes:
* The (probably unintentional) hack to handle paths as tarballs has
been removed. This is almost certainly not what users expect and is
inconsistent with flakeref handling everywhere else.
* The hack to support scp-style Git URLs has been moved to the Git
fetcher, so it's now supported not just by fetchTree but by flake
inputs.
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/
```
