context: drop support for looking up context by ambiguous changeid (API) This removes support for using the changectx constructor (and thereby repo[x]) for looking up contexts by a stringified int, a namespace key (e.g. a bookmark), or a partial hex nodeid. This means that e.g. repo[<hex nodeid>] will now fail even if a bookmark with the same name exists (which is a good thing IMO). It also means that doing repo[<non-existent node>] no longer ends up loading namespaces (which was a surprising side-effect of creating of failing to create a context object that I recently ran into while debugging something unrelated to this series). Differential Revision: https://phab.mercurial-scm.org/D3449