fix an issue where 'git fetch' would error or hang against gotd If Git has more than 16 have-lines to send it will send a flush-pkt followed by more have-lines. Due to a misunderstanding on my part, gotd didn't like this because it assumed that the flush-pkt terminates the list. Add a test coverage in a new file which we can use to test Git interop issues. Fixes a problem seen by Thomas Adam upon git fetch from got.g.o.