tests: Mercurial hooks must use ui.status for messages sent to the client

Mercurial changed so sys.stdout and sys.stderr now are intercepted instead of
being sent to the client. That caused a regression that
manual_test_vcs_operations.py test_push_new_file_hg caught - we no longer
reported 'Repository size' to the user.

The issue was introduced by a Mercurial change, but the fix is backwards
compatible with Mercurial 2.9.

The fix is to use ui.status everywhere. ui is a Mercurial thing, but
handle_git_receive also creates a ui object for Git hooks so ui.status can be
used there too.