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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.