pack-protocol: clarify LF-handling in PKT-LINE()

pack-protocol: clarify LF-handling in PKT-LINE()

The spec is very inconsistent about which PKT-LINE() parts
of the grammar include a LF. On top of that, the code is not
consistent, either (e.g., send-pack does not put newlines
into the ref-update commands it sends).

Let's make explicit the long-standing expectation that we
generally expect pkt-lines to end in a newline, but that
receivers should be lenient. This makes the spec consistent,
and matches what git already does (though it does not always
fulfill the SHOULD).

We do make an exception for the push-cert, where the
receiving code is currently a bit pickier. This is a
reasonable way to be, as the data needs to be byte-for-byte
compatible with what was signed. We _could_ make up some
rules about signing a canonicalized version including
newlines, but that would require a code change, and is out
of scope for this patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>