changegroup: don't try to build changelog chunks if not required When we extend a narrow clone without ellipsis, we don't download changelog information because that's already present with the client. However we still try to build that chunk stream. Building that chunk stream involves calling a lookup function and store.emitrevisions() API. The lookup function is called len(cl) number of times. On large repositories, where len(cl) is in millions, calling that lookup function is not a good idea. Also it's not required to use the store.emitrevisons() API because we already have nodes present which we can use. This patch short-circuits state building logic if we are processing a non-ellipsis case and changelog is not required. This saves up ~20 seconds on our internal repo for a single extend call. Differential Revision: https://phab.mercurial-scm.org/D5733