log: slowpath: do not read the full changelog When in the slowpath, we are examining _all_ changesets in revs. We need to order reads so they happen increasingly for I/O performance. Increasing windows were used to read changelog backwards in a windowed manner, reading the changelog forward inside each window. But since no revision range was specified, it was equivalent to reading the full changelog, even if a single revision was passed to the commandline. When --removed is used, we _need_ to scan all changesets, but if we're only looking for file patterns, this is not necessary and we can stick to the revspec that was given to us.