Fix regression.py when running tests in the same test
directory more than once, and the --tag differs.  If
the boost_root directory already exists, add a new
remote for a different branch (remote set-branches)
and added an explicit checkout on this branch.

If you run run.py script once with --tag=master,
and again with --tag=develop, the script will now
correctly checkout develop the second time and
will run tests using the develop fork rather than
master.