In 3c5668edd2 I mistakenly moved the check
the length of an encoded `wire::Message` from its `impl wire::Encode` to
the `fn serialize`, so that it would not apply only to messages (as
originally intended), but to *any* argument of `serialize`.
This is not a problem for gossip messages, but catastrophic for Git
streams, which tend to send a lot of data.
The fix is simple: Move the check back into `impl Encode for Message`.
a8426dfdac