commit: increase perf by avoiding unnecessary filteredrevs check
When commiting to a repo with lots of history (>400000 changesets)
the filteredrevs check (added with 5c89e7fa5bc2) in changelog.py
takes a bit of time even if the filteredrevs set is empty. Skipping
the check in that case shaves 0.36 seconds off a 2.14 second commit.
A 17% gain.