context: move handling of filtering error to revsymbol() (API) When changectx's constructor runs into various Filtered*Error, it creates an exception with a hint about using --hidden. This only makes sense when the revision was provided by the user (if we get e.g. a FilteredLookupError from repo[p1], then it's instead a programming error). Thus, I'm moving the handling into revsymbol(). Also changed "unfilteredrepo[changeid]" to "revsymbol(unfilteredrepo, changeid)" as part of the move. Differential Revision: https://phab.mercurial-scm.org/D3143