rust: add comment about lack of wdirrev handling
If hg is compiled with rust support, 'only(wdir())' crashed as
"rustext.GraphError: ('ParentOutOfRange', 2147483647)", which should instead
say "abort: working directory revision cannot be specified."
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL