tests/helper-runtests.sh

tests: avoid grafting the same change over and over

The test case added in a1381eea7c7d (graft: do not use `.remove` on a
smart set (regression), 2014-04-28) added a test case that grafted the
same change (renaming 'a' to 'b') three times over. It had description
"graft works on complex revset", but AFACT, all that it cared about
was that some ancestor of the working copy was in the set of revisions
to graft. So this patch changes the test to do that instead.

(I plan to later make it so that grafting these renames on top of each
won't create the empty commits they currently create.)

Differential Revision: https://phab.mercurial-scm.org/D7804

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL