context: rename troubles into instabilities Rename troubles context method into instabilities. Copy the old troubles method and add a deprecation warning. This way extensions calling troubles will see the deprecation warning but will not break due to new return values. The renaming is done according to https://www.mercurial-scm.org/wiki/CEDVocabulary. Differential Revision: https://phab.mercurial-scm.org/D238