move vcf-merge test data to subdir
trailing whitespace
add test data for per-alt list merging
remove some old dead code
take care about when GZipLineSource::operator bool

before, operator bool() returned false on eof which doesn't match the
behavior of standard istreams. now, it won't return false until after
the first call that actually gets no new data.
add per-alt-delimited-list to info merge strategy