extdiff: remove dir2root and pass full path as dir2 in _runperfilediff() The only use of `dir2root` was to join with `dir2` to generate the path for other side of diff. Like in previous patch, `dir1a` and `dir1b` are full paths and no longer base names, hence we pass `dir2` as full path too and making `dir2root` unrequired. Differential Revision: https://phab.mercurial-scm.org/D8970