format-patch: track several references

format-patch: track several references

Currently, format-patch can only track a single reference (the
In-Reply-To:) for each mail.  To ensure proper threading, we should
list all known references for every mail.

Change the rev_info.ref_message_id field to a string_list, so that we
can append references at will, and change the output formatting
routines to print all of them in the References: header.  The last
entry in the list is implicitly assumed to be the In-Reply-To:, which
gives output consistent with RFC 2822:

   The "References:" field will contain the contents of the parent's
   "References:" field (if any) followed by the contents of the
   parent's "Message-ID:" field (if any).

Note that this is just preparatory work; nothing uses it yet, so all
"References:" fields in the output are still only one deep.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>