tests/test-check-help.t

tests: always access the mercurial repo through `helpers-testrepo.sh`

In some contexts the mercurial repo needs to be accessed through system hg.
That's what `helpers-testrepo.sh` enforces, but some tests incorrectly
use the mercurial repo without going through that script.

This patch fixes those tests.

#require test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"

  $ cat <<'EOF' > scanhelptopics.py
  > import re
  > import sys
  > if sys.platform == "win32":
  >     import msvcrt
  >     import os
  >     msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
  > stdout = getattr(sys.stdout, 'buffer', sys.stdout)
  > topics = set()
  > topicre = re.compile(br':hg:`help ([a-z0-9\-.]+)`')
  > for fname in sys.argv:
  >     with open(fname, 'rb') as f:
  >         topics.update(m.group(1) for m in topicre.finditer(f.read()))
  > for s in sorted(topics):
  >     stdout.write(b'%s\n' % s)
  > EOF

Check if ":hg:`help TOPIC`" is valid:
(use "xargs -n1 -t" to see which help commands are executed)

  $ cd "$TESTDIR"/..
  $ testrepohg files 'glob:{hgdemandimport,hgext,mercurial}/**/*.py' \
  > | sed 's|\\|/|g' \
  > | xargs "$PYTHON" "$TESTTMP/scanhelptopics.py" \
  > | xargs -n1 hg --cwd "$TESTTMP" help --config extensions.phabricator= > /dev/null