tests/helper-runtests.sh

rust: Use `&HgPath` instead of `&HgPathBuf` in may APIs

Getting the former (through `Deref`) is almost the only useful thing one can
do with the latter anyway. With this changes, API become more flexible for the
"provider" of these paths which may store something else that Deref’s to HgPath,
such as `std::borrow::Cow<HgPath>`. Using `Cow` can help reduce memory alloactions
and copying.

Differential Revision: https://phab.mercurial-scm.org/D10558

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL