ui: don't fixup [paths] sub-options
As part of developing a subsequent patch I discovered that sub-option
values like "." were getting converted to paths. This is because the
[paths] section is treated specially during config loading.
This patch prevents post-processing sub-options from the [paths]
section.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL