context: rename troubled into isunstable As we changed the meaning of unstable between the old vocabulary and the new one, we can't reuse the unstable method name at the risk of breaking extensions calling unstable and getting a wrong result. Instead rename troubled into isunstable so extensions will continue to work. The renaming is done according to https://www.mercurial-scm.org/wiki/CEDVocabulary. Differential Revision: https://phab.mercurial-scm.org/D242