Iterating over the current set of file names may not work when no files are
given (in which case all agenda files should be visisted). However, instead of
fixing the set of files, it should be more robust to find all referenced buffers
by iterating over the markers of the entries. In that case, even if a file is
visited by multiple buffers, we can be sure that the right buffers are referesh
before the timeline is redrawn.