Mercurial 6 doesn't have the httpclient directory any more, so the
installer can ignore it.

This reverts to commit 1582036,
discarding everything done on this branch since that commit.

This reverts commit 317ff12 and goes
back to Mercurial 6.

Core code was using version 11.0.0-beta0077 but tests were still on
LCModel version 10. Update tests to use 11.0.0-beta0077 as well.

TargetDir is defined by Visual Studio but not by the dotnet CLI tools

This workflow uses `dotnet test` and `dotnet build` steps directly in
the GHA runner, rather than building an LfMerge builder image. That way
it's a lot easier to understand the build process, and to replicate it
on dev machines (just do `dotnet build` and `dotnet test`).

dotnet clean can sometimes fail if, for example, you're switching from
targeting net6.0 to net8.0. That's not a problem, so we let the script
continue if the `dotnet clean` step fails.

Now that ChorusMerge builds for net8.0, we should copy its net8.0
runtimeconfig.json to our build output, not the older net6.0 one.

Mercurial 6 needs hgdemandimport to live alongside hgext.

This will allow us to more easily run a local build and deploy it to
local LF for testing, without having to wait for a GHA workflow.

Now that we know the GHA workflow works, we no longer need to run it
on pushes to the test branch. We can simply run it on pull requests and
that's it.

We used to need to upload a tarball because GitHub Actions wasn't
preserving Unix file permissions in the .zip file it would upload, so
things like executable bits were getting stripped. Now that there's an
upload-artifact@v4 that works on different principles internally, let's
see if this step is still necessary. If this procudes Docker images that
still run, we might be able to save some build time.

We also comment out the `dotnet test` step here to save 7 minutes of
waiting for the build; the affected code is after the `dotnet test` so
it will continue to pass but tell us nothing about the changes here.

Will we need it in the release step? Let's leave it out for now and see
what happens.

GitHub Actions upload-artifact still strips executable bits, so we still
need the step of creating a .tar.gz file.