config: introduce a new value for ui.relative-paths getting old behavior The few places I've modified so far to respect ui.relative-paths have traditionally defaulted showing the path from the repo root. However, some commands (at least `hg files`) default to showing paths relative to the cwd. Let's allow a special value for ui.relative-paths to preserve the old behavior, so we can use that as default value for it. I don't expect that anyone would want to set this value, so perhaps we could have relied on it being unset, but I don't really like behaviors that can only be achieved by a unset config option. Differential Revision: https://phab.mercurial-scm.org/D5800