diff: respect ui.relative-paths for warning about path outside --root

Authored by martinvonz.

Description

diff: respect ui.relative-paths for warning about path outside --root

I set legacyrelativevalue=True rather than being based on what
patterns the user passed (as the current match.uipath() does). I think
it simply doesn't really matter for this message (there are not even
any tests for it).

Differential Revision: https://phab.mercurial-scm.org/D5905