changelog: lazy decode description (API) Currently, changelog reading decodes read values. This is wasteful because a lot of times consumers aren't interested in some of these values. This patch changes description decoding to occur in changectx as needed. revsets reading changelog entries appear to speed up slightly: revset #7: author(lmoscovicz) plain 0) 0.906329 1) 0.872653 revset #8: author(mpm) plain 0) 0.903478 1) 0.878037 revset #9: author(lmoscovicz) or author(mpm) plain 0) 1.817855 1) 1.778680 revset #10: author(mpm) or author(lmoscovicz) plain 0) 1.837052 1) 1.764568