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