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

Test issue2761

  $ hg init

  $ touch to-be-deleted
  $ hg add
  adding to-be-deleted
  $ hg ci -m first
  $ echo a > to-be-deleted
  $ hg ci -m second
  $ rm to-be-deleted
  $ hg diff -r 0

Same issue, different code path

  $ hg up -C
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ touch does-not-exist-in-1
  $ hg add
  adding does-not-exist-in-1
  $ hg ci -m third
  $ rm does-not-exist-in-1
  $ hg diff -r 1