manifestmerge: fix order in which manifests are fetched If the manifest of an earlier revision on the same delta chain is read before that of a later revision, the revlog remembers that we parsed the earlier revision and continues applying deltas from there onwards. If manifests are parsed the other way round, we have to start over from the fulltext. For a fresh clone of mozilla-central, updating from 29dd80c95b7d to its parent aab96936a177 requires approximately 400 fewer zlib.decompress calls, which results in a speedup from 1.10 seconds to 1.05.