BaseReceivePack: Treat all LFs as optional

BaseReceivePack: Treat all LFs as optional

Discussion on the git mailing list has concluded[1] that the intended
behavior for all (non-sideband) portions of the receive-pack protocol
is for trailing LFs in pkt-lines to be optional. Go back to using
PacketLineIn#readString() everywhere.

For push certificates specifically, we agreed that the payload signed
by the client is always concatenated with LFs even though the client
MAY omit LFs when framing the certificate for the wire. This is still
reflected in the implementation of PushCertificate#toText().

[1] http://thread.gmane.org/gmane.comp.version-control.git/273175/focus=273412

Change-Id: I817231c4d4defececb8722142fea18ff42e06e44