bookmarks: introduce a repo._bookmarks.changectx(mark) method and use it Many places were doing repo[mark], which usually works, but it's slightly incorrect: if the bookmark has a name that matches a full hex nodeid of another node, then the context for the other node will be returned instead. Also, I'm about to remove support for repo[<namespace thing>] :) Differential Revision: https://phab.mercurial-scm.org/D3162