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
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support* (glob)
Ran 1 tests, 1 results as expected* (glob)