send-email: only consider lines containing @ or <> for automatic Cc'ing

send-email: only consider lines containing @ or <> for automatic Cc'ing

While the address sanitizations routines do accept local addresses, that
is almost never what is meant in a Cc or Signed-off-by trailer.

Looking through all the signed-off-by lines in the linux kernel tree
without a @, there are mostly two patterns: Either just a full name, or
a full name followed by <user at domain.com> (i.e., with the word at
instead of a @), and minor variations. For cc lines, the same patterns
appear, along with lots of "cc stable" variations that do not actually
name stable@vger.kernel.org

  Cc: stable # introduced pre-git times
  cc: stable.kernel.org

In the <user at domain.com> cases, one gets a chance to interactively
fix it. But when there is no <> pair, it seems we end up just using the
first word as a (local) address.

As the number of cases where a local address really was meant is
likely (and anecdotally) quite small compared to the number of cases
where we end up cc'ing a garbage address, insist on at least a @ or a <>
pair being present.

This is also preparation for the next patch, where we are likely to
encounter even more non-addresses in -by lines, such as

  Reported-by: Coverity
  Patch-generated-by: Coccinelle
Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>