revlog: overwrite revlog config through copy of the config object
The new objects allow for this kind of blanket approach that make things
cleaner.
If we have more cases, it would probably deserve a context manager, but since we
only have two usage, I don't think it is worth it.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL