dispatch: pass root path in ui.readconfig() as root of main repo
Since we are reading main (shared-source) repository config options, we
should pass root as that repository root only.
Differential Revision: https://phab.mercurial-scm.org/D9359
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL