diffhelper: rename module to avoid conflicts with ancient C module (issue5846)
Historically we had had C extensions in mercurial/, which shadows the pure
Python modules of the same name forever unless we do clean build/install.
I'm sloppy to think about new name, so just dropped the "s".
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL