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 ()
class='active' href='/guix/log/gnu/installer/proxy.scm'>log</a><a href='/guix/tree/gnu/installer/proxy.scm?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>tree</a><a href='/guix/commit/gnu/installer/proxy.scm?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>commit</a><a href='/guix/diff/gnu/installer/proxy.scm?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/installer/proxy.scm'>
<input type='hidden' name='id' value='3ecf9d44bb927fc88223a15431428cb741dd1c2c'/><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=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>root</a>/<a href='/guix/log/gnu?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>gnu</a>/<a href='/guix/log/gnu/installer?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>installer</a>/<a href='/guix/log/gnu/installer/proxy.scm?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c'>proxy.scm</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/gnu/installer/proxy.scm?id=3ecf9d44bb927fc88223a15431428cb741dd1c2c&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-11 21:37:07 +0100'>2020-12-11</span></td><td><a href='/guix/commit/gnu/installer/proxy.scm?id=8361817bf693742757b096468198626f297bb09e'>install: Discover local substitute servers.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/substitutes.scm: New file.
* gnu/installer/newt/substitutes.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add them.
* po/guix/POTFILES.in: Add gnu/installer/newt/substitutes.scm.
* gnu/installer/proxy.scm (with-silent-shepherd): Move to ...
* gnu/installer/utils.scm: ... here.
* gnu/installer/record.scm (&lt;installer&gt;)[substitutes-page]: New field.
* gnu/installer/newt.scm (substitutes-page): New procedure,
(newt-installer): register it.
* gnu/installer.scm (installer-steps): Add "substitutes-page" step.
* gnu/system/install.scm (%installation-services): Add avahi-service-type and
enable substitute server discover in guix-service-type.
[&lt;name-service-switch&gt;]: Set it to %mdns-host-lookup-nss.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-04-08 10:24:02 +0200'>2020-04-08</span></td><td><a href='/guix/commit/gnu/installer/proxy.scm?id=ee0ad7803c82383ac1bced073d8670bc407797e7'>installer: Add proxy support.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/proxy.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add it.
* po/guix/POTFILES.in: Add it.
* gnu/installer/newt/parameters.scm (run-proxy-page): New procedure,
(run-parameters-page): add the previous procedure to the parameters menu.
</span></span></td><td>Mathieu Othacehe</td></tr>