merger: fix chunk cache never getting cleared (#4531)

Commit 7fc13c5 indroduced less aggressive cache invalidation for the
chunk cache but saved the new revision before comparing it with the old
one, and so the cache was never considered invalid.

Fixes #4529

src/matcher.go

@@ -85,11 +85,11 @@ func (m *Matcher) Loop() {
 		cacheCleared := false
 		if request.sort != m.sort || request.revision != m.revision {
 			m.sort = request.sort
-			m.revision = request.revision
 			m.mergerCache = make(map[string]*Merger)
 			if !request.revision.compatible(m.revision) {
 				m.cache.Clear()
 			}
+			m.revision = request.revision
 			cacheCleared = true
 		}