tests/test-dirstate-nonnormalset.t

run-tests: stop writing a `python3` symlink pointing to python2

Having `python3` actually pointing to `python2` is bad. So we stop doing so.

In addition we need to re-introduce a `python` executable since some of the
script really need to be able to say "current python" in their shbang. For
example, `hghave` is one of such script.

The faulty changes where introduced by c102b704edb5.

Differential Revision: https://phab.mercurial-scm.org/D10943

  $ cat >> $HGRCPATH << EOF
  > [command-templates]
  > log="{rev}:{node|short} ({phase}) [{tags} {bookmarks}] {desc|firstline}\n"
  > [extensions]
  > dirstateparanoidcheck = $TESTDIR/../contrib/dirstatenonnormalcheck.py
  > [experimental]
  > nonnormalparanoidcheck = True
  > [devel]
  > all-warnings=True
  > EOF
  $ mkcommit() {
  >    echo "$1" > "$1"
  >    hg add "$1"
  >    hg ci -m "add $1"
  > }

  $ hg init testrepo
  $ cd testrepo
  $ mkcommit a
  $ mkcommit b
  $ mkcommit c
  $ hg status