Previously, we flushed intermediate data out of the underlying *bufio.Writer
used by the implementation of `pkt-line` in `*git.pktline` after _every_ write.
This means that we would flush after partial packets, whole packets, and after
pkt-line flush sequences. This is unnecessary.
Let us instead flush only after an entire packet has been written, after
writing the `0000` flush sequence.
/ref git-lfs/git-lfs#1640
