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
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h