tests: use "hg help revisions.<predicate>" instead of grepping
We have specific syntax for displaying the help text for a particular
revset predicate, so use that instead of grepping through the full
output.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL