there was a diff that went in a while ago to improve
this, but it got backed out because it encounters a
bug in upstream git -- the spec says that a single
ACK should be sent when not using multi-ack modes,
but they send back multiple ones.
This commit brings back the functionality, and works
around the upstream git bug in two different ways.
First, it skips the packets up until it finds the
start of a pack header.
Second, it deduplicates the want messages, which
is what seems to trigger the duplicate ACKs that
cause us trouble.
261d1ac0e3