match: de-flake test-doctest.py by not depending on util.dirs() order util.dirs() yields directories in arbitrary order, which has made test-doctest.py flaky. I think they have been flaky since d8e55c0c642c (util: make util.dirs() and util.finddirs() include root directory (API), 2017-05-16). Before that commit, I think util.dirs() would return at most one entry, so there was only one iteration order. This patch fixes the problem by making _rootsdirsandparents() return a set (whose __str__() is defined to be in sorted order, I believe). The only caller wanted a set anyway. Differential Revision: https://phab.mercurial-scm.org/D6432

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