send-email: avoid incorrect header propagation

send-email: avoid incorrect header propagation

If multiple independent patches are sent with send-email, even if the
"In-Reply-To" and "References" headers are not managed by --thread or
--in-reply-to, their values may be propagated from prior patches to
subsequent patches with no such headers defined.

To mitigate this and potential future issues, make sure all global
patch-specific variables are always either handled by
command-specific code (e.g. threading), or are reset to their default
values for every iteration.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Marvin Häuser <mhaeuser@posteo.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>