changegroup: drop special-casing of flat manifests Since c08814b48ae5 (changegroup: avoid iterating the whole manifest, 2015-12-04), the manifest linkrev callback iterates over only the files that were touched according the the changeset. Before that change, we iterated over all files returned in manifest.readfast(). That method returns the files in the delta, if the delta parent is a parent, otherwise it returns the full manifest. Most manifest revisions end up using one of the parents as its delta parent, so most of the time, the method returns a short manifest. It seems that that happens often enough that it doesn't really matter; I could not reproduce the timings reported in that change. Since the treemanifest code now works quite differently, and since that code also works correctly for flat manifests, let's drop the special-casing of flat manifests.