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.

When reading clients close the connection early do not ignore this condition in session_read. The code removed here intended to handle the case of a writing client uploading a pack file, and thus does not belong into session_read.c. I overlooked this when splitting session.c into two files. This might fix an issue observed on got.g.o where the anonymous user was no longer connected and left lingering session_read and repo_read processes hanging around until the request timeout was reached.

This makes it easier to tweak the read/write code paths separately.