Preserve subject when replying to patch threads.
Upstream status: likely to be in the next release (bug#66330)

diff --git a/debbugs-gnu.el b/debbugs-gnu.el
index f366fba11d..cd549e364d 100644
--- a/debbugs-gnu.el
+++ b/debbugs-gnu.el
@@ -1747,6 +1747,12 @@ MERGED is the list of bugs merged with this one."
 	 (format "Re: bug#%d: %s" id (alist-get 'subject status)))
     (debbugs-gnu-summary-mode 1)))

+(defcustom debbugs-gnu-summary-keep-subject
+  (rx "[PATCH" (? (0+ (not (any digit "/]"))) (1+ digit) "/" (1+ digit)) "]")
+  "Regular expression which keeps the original message subject in replies."
+  :version "29.1"
+  :type 'regexp)
+
 (defvar debbugs-gnu-summary-mode-map
   (let ((map (make-sparse-keymap)))
     (define-key map "C" #'debbugs-gnu-send-control-message)
@@ -1778,6 +1784,8 @@ MERGED is the list of bugs merged with this one."
 			    (cons new new))
 			address))))))
 	  ,@(and debbugs-gnu-subject
+                 (not (string-match-p debbugs-gnu-summary-keep-subject
+                                      debbugs-gnu-subject))
 		 `((subject ,debbugs-gnu-subject)))))))

 (defun debbugs-gnu-guess-current-id ()
 href='/guix/log/doc/htmlxref.cnf'>log</a><a href='/guix/tree/doc/htmlxref.cnf?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'>tree</a><a href='/guix/commit/doc/htmlxref.cnf?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'>commit</a><a href='/guix/diff/doc/htmlxref.cnf?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/doc/htmlxref.cnf'>
<input type='hidden' name='id' value='3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'>root</a>/<a href='/guix/log/doc?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'>doc</a>/<a href='/guix/log/doc/htmlxref.cnf?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9'>htmlxref.cnf</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/doc/htmlxref.cnf?id=3e0902cbcc6acf0079283f7f8e6afdbe66f81fc9&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-06-16 21:52:49 -0400'>2022-06-16</span></td><td><a href='/guix/commit/doc/htmlxref.cnf?id=ec7a75034421654d2ece2ba20033b44d345547f0'>doc: Remove obsolete comment from htmlxref.cnf.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is follow up to &lt;https://issues.guix.gnu.org/55290&gt;.

* doc/htmlxref.cnf (geiser): Delete obsolete comment.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-04-08 19:59:25 +0200'>2022-04-08</span></td><td><a href='/guix/commit/doc/htmlxref.cnf?id=868da34d54365023223a4ff7520043ba55ad64e8'>doc: Fix cookbook URLs in htmlxref.cnf.</a><span class='msg-avail'>...<span class='msg-tooltip'>Reported by Greg Hogan &lt;code@greghogan.com&gt;.

* doc/htmlxref.cnf (GUIX_ROOT): New variable.
(GUIX, GUIX_COOKBOOK): Adjust.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-10-04 16:36:58 +0200'>2021-10-04</span></td><td><a href='/guix/commit/doc/htmlxref.cnf?id=1267c2e9efdb6e324ce1c7b1fcef242f8208a61f'>doc: Update htmlxref.cnf.</a><span class='msg-avail'>...<span class='msg-tooltip'>Update and fix broken references to cuirass, git, and guix-cookbook.

* doc/htmlxref.cnf: Update from Texinfo.  Factorize Guix manuals.
(cuirass, git, guix-cookbook, guix-cookbook.de, guix-cookbook.fr): New
entries.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Sarah Morgensen</td></tr>
<tr><td><span title='2021-04-24 16:46:22 +0200'>2021-04-24</span></td><td><a href='/guix/commit/doc/htmlxref.cnf?id=1ebd086286c3825cfae31e75f8394986f1a18736'>doc: Fix cross-reference URL to translated manual.</a><span class='msg-avail'>...<span class='msg-tooltip'>* doc/htmlxref.cnf: Fix translated manual URL.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2020-01-12 00:19:10 +0100'>2020-01-12</span></td><td><a href='/guix/commit/doc/htmlxref.cnf?id=30d83f445b70ebce7f01eaa91c8f418ff0a98321'>doc: Update htmlxref.cnf.</a><span class='msg-avail'>...<span class='msg-tooltip'>* doc/htmlxref.cnf: Update from Texinfo.  Add entries for "mes",
"guix.ru", and "guix.zh_CN".
</span></span></td><td>Ludovic Courtès</td></tr>