windows: don't consider '$$' to be an escaped '$' when translating to cmd.exe
This functionality was inherited from `os.path.expandvars()`. But the point of
adding this translating code is to be able to write a portable hook, and bash
wouldn't replace '$$' with '$'. Escaping with '\' works, and is portable.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL