repoview: avoid wrapping changelog if there's nothing to filter
This simplifies the code a bit by moving the optimizaton for no
filtered revisions to one place. I assume it also makes working with
repos without obsmarkers a little faster, but it doesn't seem
significant.
Differential Revision: https://phab.mercurial-scm.org/D7248
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273ce12ad8f1` will report some style violations on a very old commit).