bug#49765: 27.1; add a show mime tree command

[積丹尼 Dan Jacobson]

Mutt's has a handy v command that shows the mime structure of a message:
  I     1 <no description> [multipa/alternativ, 7bitC?, >, 55K]
  I     2 ├─><no description> [text/plain, quotedC?, utf-8>, 2.1K]
  I     3 └─><no description> [text/html, quotedC?, utf-8>, 52K]

Gnus has commands to buttonize, etc. But none show the tree structure.


OK, Gnus has C-d: gnus-summary-enter-digest-group, which shows:

 .  1 210727||台電電子帳單服:<* mixed> 台電e-Bill 110年7月電費繳費憑證(電號：0748730等)
 .  1 210727||台電電子帳單服:<1.* alternative>
 .  1 210727||台電電子帳單服:<1.1 text>
 .  1 210727||台電電子帳單服:<1.2 html>
 .  1 210727||台電電子帳單服:<2 pdf> "07487301100723RD.pdf"
 .  1 210727||台電電子帳單服:<3 pdf> "07488871100723RD.pdf"
 .  1 210727||台電電子帳單服:<4 pdf> "領獎收據聯.pdf"

Which apparently relies on numbers to indicate the structure.
Nice, but maybe indenting could also be used.

Also it seems this C-d command was not intended to show mime structures,
and I am just lucky it did for this message. So a specialized command is
needed.

Else sometimes it just shows:
 .  1 210727||DreamHost     :You to your 9-to-5 job: “We need to talk…”
 .  1 210727||DreamHost     :You to your 9-to-5 job: “We need to talk…”
 .  1 210727||DreamHost     :You to your 9-to-5 job: “We need to talk…”
(For the message at the top of this bug report.)