send-email: handle multiple Cc addresses when reading mbox message

send-email: handle multiple Cc addresses when reading mbox message

When git format-patch is given multiple --cc arguments, it generates a
Cc header that looks like:

 Cc: first@example.com,
     second@example.com,
     third@example.com

Before this commit, send-email was unable to handle such a message as it
did not handle folded header lines, nor multiple recipients in a Cc
line.

This patch:

- Unfolds header lines by pre-processing the header before extracting
  any of its fields.

- Handles Cc lines with multiple recipients.

- Adds use of Mail::Address if available for splitting Cc line and
  the "Who should the emails be sent to?" prompt", with fall back to
  existing split_addrs() function.

- Tests the new functionality and adds two tests for detecting whether
  "From:" appears correctly in message body when patch author differs
  from patch sender.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>