tests/helper-runtests.sh

graft: correct documentation about options can be reapplied

Now options --user and --date can be reapplied after we hit a conflict.

Differential Revision: https://phab.mercurial-scm.org/D3678

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL