Bug-reference auto-setup for Rmail (please test)

[Tassilo Horn <tsdh@gnu.org>]

Hi all,

I've just pushed a commit which should make bug-reference auto-setup
work for Rmail messages.  I'm no Rmail user, so I'd appreciate some
testing from actual users.

What you need to do is activate `bug-reference-mode' in Rmail, so
probably:

  (add-hook 'rmail-mode-hook #'bug-reference-mode)

and then it'll try to guess the right `bug-reference-bug-regexp' and
`bug-reference-url-format' based on the List-Id, To, From, and Cc
headers according to the customize option
`bug-reference-setup-from-mail-alist' which should at least have a
suitable default value for (some) emacs-related mailinglists including
emacs-devel.

There is one problem where I need implementation advice.  The
bug-reference auto-setup is a no-op if `bug-reference-bug-regexp' and
`bug-reference-url-format' are already set (see
`bug-reference--run-auto-setup').  So if you view an emacs-devel message
first, it'll set those variables.  When you move on to the next message
(which might be completely unrelated to emacs), the variables are still
set, so no new guesswork will be done.

We'd need to nuke the buffer-local values and re-enable
`bug-reference-mode' on switching messages, e.g.,

  (add-hook 'rmail-show-message-hook #'rmail-bug-reference-setup)

for a yet to be written rmail-bug-reference-setup which is a bit sad
because for all other modes where I've added this auto-setup capability
(Gnus, Mu4e [currently only in my ~/.emacs], rcirc, ERC, VC) the
"normal"

  (add-hook '<mode>-hook #'bug-reference-mode)

will do, that is, for Rmail the user would need to use a special
function rather than just enabling `bug-reference-mode' in the mode of
his choice.  (Of course, this could be documented.)

Alternatively, Rmail itself could check if `bug-reference-mode' is
enabled, nuke the two variables, and call
`bug-reference--run-auto-setup'.

WDYT?

Bye,
Tassilo