git-send-email: Generalize auto-cc recipient mechanism.

git-send-email: Generalize auto-cc recipient mechanism.

There are a few options to git-send-email to suppress the automatic
generation of 'Cc' fields: --suppress-from, and --signed-off-cc.
However, there are other times that git-send-email automatically
includes Cc'd recipients.  This is not desirable for all development
environments.

Add a new option --suppress-cc, which can be specified one or more
times to list the categories of auto-cc fields that should be
suppressed.  If not specified, it defaults to values to give the same
behavior as specified by --suppress-from, and --signed-off-cc.  The
categories are:

  self   - patch sender.  Same as --suppress-from.
  author - patch author.
  cc     - cc lines mentioned in the patch.
  cccmd  - avoid running the cccmd.
  sob    - signed off by lines.
  all    - all non-explicit recipients

Signed-off-by: David Brown <git@davidb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>