Fix a regression in 6.8 where the 'single' headings option was not recognized.

Taken from upstream:

  https://git.savannah.gnu.org/cgit/texinfo.git/commit/?id=13a8894fe2faa45b04033d7122a8fe7939ce6aa2

diff --git a/tp/Texinfo/XS/parsetexi/end_line.c b/tp/Texinfo/XS/parsetexi/end_line.c
index 4556780052..3fc2065616 100644
--- a/tp/Texinfo/XS/parsetexi/end_line.c
+++ b/tp/Texinfo/XS/parsetexi/end_line.c
@@ -778,8 +778,8 @@ kdbinputstyle_invalid:
     case CM_headings:
       {
         if (!strcmp (line, "off") || !strcmp (line, "on")
-            || !strcmp (line, "double") || !strcmp (line, "singleafter")
-            || !strcmp (line, "doubleafter"))
+            || !strcmp (line, "single") || !strcmp (line, "double")
+            || !strcmp (line, "singleafter") || !strcmp (line, "doubleafter"))
           {
             ADD_ARG(line);
           }
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<tr><td><span title='2022-01-14 21:47:42 +0100'>2022-01-14</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=5d9d1d9a7d2f1bba6623eda0bab664ea9ecb21ce'>gnu: Add lukesmithxyz-st.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (lukesmithxyz-st): New variable.

Signed-off-by: Nicolas Goaziou &lt;mail@nicolasgoaziou.fr&gt;
</span></span></td><td>jgart</td></tr>
<tr><td><span title='2021-12-26 19:22:35 +0100'>2021-12-26</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=ea780316b0d8063d58994ca37bb503204b0f3af5'>gnu: Add xst.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (xst): New variable.

Co-authored-by: Raghav Gururajan &lt;rg@raghavgururajan.name&gt;
Signed-off-by: Mathieu Othacehe &lt;othacehe@gnu.org&gt;
</span></span></td><td>jgart</td></tr>
<tr><td><span title='2021-12-17 09:15:57 +0100'>2021-12-17</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=96010ba902b8241b311f3ce2c5cc2bc515db99a7'>gnu: Add sfeed.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (sfeed): New variable.

Signed-off-by: Nicolas Goaziou &lt;mail@nicolasgoaziou.fr&gt;
</span></span></td><td>Nikolay Korotkiy</td></tr>
<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-13 11:49:15 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=1052ae5f03de931b52c7a638c8e4f8d8d7093af3'>Merge branch 'master' into core-updates-frozen</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-10 23:34:36 +0100'>2021-12-10</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=f48911635217bdff4a313506895b4cbaf09e7d65'>gnu: Add sfm.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (sfm): New variable.

Signed-off-by: Nicolas Goaziou &lt;mail@nicolasgoaziou.fr&gt;
</span></span></td><td>Nikolay Korotkiy</td></tr>
<tr><td><span title='2021-12-05 19:17:41 +0100'>2021-12-05</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=9bc0f45df5d6aed217020b1183dca54989844fb0'>Merge remote-tracking branch 'origin/master' into core-updates-frozen</a></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2021-12-03 15:27:41 -0500'>2021-12-03</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=19528e39bb00cc25a41a134f7af63873fa297b77'>gnu: st: Install terminfo files.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (st)[native-inputs]: Add ncurses.
[make-flags](TERMINFO): New flag.
[phases](inhibit-terminfo-install): Remove phase.

Signed-off-by: Raghav Gururajan &lt;rg@raghavgururajan.name&gt;
</span></span></td><td>jgart</td></tr>
<tr><td><span title='2021-11-27 10:47:34 +0100'>2021-11-27</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=4967eb97b5f2067e8ea512ad6e87743292859c3c'>gnu: surf: Build with libsoup2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (surf)[inputs]: Replace webkitgtk with
  webkitgtk-with-libsoup2.
</span></span></td><td>Guillaume Le Vaillant</td></tr>
<tr><td><span title='2021-10-31 14:49:47 +0200'>2021-10-31</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=bc5155b952ae8bdbc56aded4d8d39768b4e2a7d4'>Merge remote-tracking branch 'origin/master' into core-updates-frozen</a></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2021-10-27 00:13:09 +0200'>2021-10-27</span></td><td><a href='/guix/commit/gnu/packages/suckless.scm?id=89d8417b371f3918f0508bbc561675ec100a6add'>gnu: snooze: Fix installation destination.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/suckless.scm (snooze)[arguments]: Remove bogus $(DESTDIR)
from #:make-flags to install in $(PREFIX).

Reported by Christopher Howard &lt;christopher@librehacker.com&gt;
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>