Add Felipe's git-marks-check to the repository, for convenience

This doesn't solve the actual issues, but helps mitigate them in many
cases.

I am not completely sure what the "actual" issues are, but one seems to
be that it is apparently relatively easy to end up with a corrupt marks
file (and indeed, the code loading / saving marks does not attempt to be
particular robust).

The other problem hinted at in issue #4 seems to  be when tips in the
remote hg repository change, then this seems to also cause troubles.

See also:
  https://github.com/felipec/git-remote-hg/issues/4
  https://github.com/felipec/git-remote-hg/issues/10
Get rid of GIT_REMOTE_HG_TEST_REMOTE and special case for local hg repos

For local hg repositories, the repo was not cloned. This special case
made it necessary to have GIT_REMOTE_HG_TEST_REMOTE for use by tests.
It also caused some problems for me, see e.g. issue #26.