Only generate date header when rendering is guaranteed

The OldChangesListRecentChangesLine hook can skip rendering of
a particular rc line.  If that line was the one that would have
added a new date header then no date header is output.  The
pushes checking for a new date header until after we know a
line will be output so none of the headers get lost.

Change-Id: I64ddd99c6af0b562802504b803563cf77fc2eb28
Merge "Only generate date header when rendering is guaranteed"