emacs: quote MML tags in replies

[Dmitry Kurochkin]

Hi Aaron.

Thanks for your work!  I took the liberty to do some cleanups for your
patch.  Below is a detailed list of changes.

Hope this helps.

Changes since v2:

* change patch names to be consistent with others:

  - s/emacs:/test:/ for the test patch

  - lower case the first word after colon in the patch title

* polish NEWS wording, move it to 0.12 section

* add comment to `mml-quote-region' call, as suggested by Tomi [1]

* fix and clean up the test:

  - set `notmuch-fcc-dirs' to nil to avoid adding the Fcc header,
    otherwise it breaks the test on other systems as pointed by
    David [2]

  - use default values for add_message parameters where possible

  - use a sane subject value in add_message

  - use shorter MML tag as produced by (mml-insert-part)

  - indenting and other minor cleanups

Regards,
  Dmitry

[1] id:"m2wr89ioos.fsf at guru.guru-group.fi"
[2] id:"87ehugzycb.fsf at zancas.localnet"