[Mutt] #3081: Allow customisation of headers stripped by mutt_prepare_template()

[Mutt <fleas@xxxxxxxx>]

#3081: Allow customisation of headers stripped by mutt_prepare_template()

 I mail out a git patch series by sending it to a mailbox, moving the
 messages into my postponed folder, and then recovering them to edit and
 send.

 git-format-patch generates a thread by setting Message-ID on the first
 message and References & In-Reply-To on the subsequent message.
 Unfortunately, mutt strips Message-ID when retrieving postponed messages,
 so the thread ends up broken.

 Please change mutt_prepare_template() so that the set of stripped headers
 can be customised.

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Ticket URL: <http://dev.mutt.org/trac/ticket/3081>