Special:Export returns an empty XML skelleton if the "current revision only"
option is not checked. This is the case no matter if the 100-revision-limit is
hit or not. I suspect that it's a problem with the code that implements that
limit, so i'm posting it as a wikimedia issue rather than a mediawiki issue.
BTW: I'm not sure what the output is supposed to look like of the max number of
revisions is hit - is it truncated, or an empty skelleton, or does it result in
an error? Failing silently does not seem a good idea, an error message would be