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

wrap 0: [0, 'orig']
wrap 1: [1, 0, 'orig']
wrap 2: [2, 1, 0, 'orig']
wrap 3: [3, 2, 1, 0, 'orig']
wrap 4: [4, 3, 2, 1, 0, 'orig']
wrap 0: [0, 4, 3, 2, 1, 0, 'orig']
unwrap 3: 3: [0, 4, 2, 1, 0, 'orig']
unwrap -: 0: [4, 2, 1, 0, 'orig']
unwrap 0: 0: [4, 2, 1, 'orig']
unwrap 4: 4: [2, 1, 'orig']
unwrap 0: -: ValueError
unwrap 2: 2: [1, 'orig']
unwrap 1: 1: ['orig']
unwrap -: -: IndexError
context manager ['orig']
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context manager [0, 1, 'orig']
context manager [2, 0, 1, 'orig']
context manager [2, 1, 'orig']
context manager [2, 'orig']
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