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.
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