This is no replacement for applying the filters later but speeds up
project retrieval noticeably for large Gitlab instances.

Also, since there is a default value for --month now, we don't need to
think about unset date filters.

We didn't do this at first but it has become clear that projects are
completely independent in Gitlab. The downside is that we now need to
make assumptions about project ordering in the data passed from fetch to
report. Reporting has become much more readable, though, so that's
probably worth it.