Add a minimum negotiation feature for fetch

Add a minimum negotiation feature for fetch

Android an Chrome have several repos with >300k refs. We sometimes see
negotiations of >100k rounds. This change provides a "minimal negotiation"
feature on the client side that limits how many "have" lines the client
sends. The client extracts the current SHA-1 values for the refs in its
wants set, and terminates negotiation early when all of those values have
been sent as haves. If a new branch is being fetched then that set will
be empty and the client will terminate after current default minimum
of two rounds.

This feature is gated behind a "fetch.useminimalnegotiation" configuration
flag, which defaults to false.

Change-Id: Ib12b095cac76a59da6e8f72773c4129e3b32ff2b
Signed-off-by: Terry Parker <tparker@google.com>