resolve: slightly simplify join expression by joining with empty strings
Differential Revision: https://phab.mercurial-scm.org/D5938
resolve: respect ui.relative-paths also for warning messages
I guess this should have been part of
72a9aacff645 (resolve: respect
ui.relative-paths, 2019-01-29).
Differential Revision: https://phab.mercurial-scm.org/D5937