Provide sane default for the @direntry

[Stefan Monnier]

When exporting to Texinfo, the patch below makes it easier to generate
a good `@direntry` by using sane defaults.
For most files, you'll just need

    #+TEXINFO_DIR_CATEGORY: {my-category}

I believe it also makes it a bit harder to shoot oneself in the foot and
generate an invalid entry (e.g. with a missing or wrong file name).


        Stefan

diff --git a/lisp/org/ox-texinfo.el b/lisp/org/ox-texinfo.el
index 84313645e6e..beea7aacab7 100644
--- a/lisp/org/ox-texinfo.el
+++ b/lisp/org/ox-texinfo.el
@@ -817,17 +799,27 @@ org-texinfo-template
                   (org-export-data copying info))))
      ;; Info directory information.  Only supply if both title and
      ;; category are provided.
-     (let ((dircat (plist-get info :texinfo-dircat))
-          (dirtitle
-           (let ((title (plist-get info :texinfo-dirtitle)))
-             (and title
-                  (string-match "^\\(?:\\* \\)?\\(.*?\\)\\(\\.\\)?$" title)
-                  (format "* %s." (match-string 1 title))))))
-       (when (and dircat dirtitle)
+     (let* ((dircat (plist-get info :texinfo-dircat))
+           (dt (plist-get info :texinfo-dirtitle))
+           (file (file-name-sans-extension
+                  (or (org-strip-quotes (plist-get info :texinfo-filename))
+                      (plist-get info :output-file))))
+           (dirtitle
+             (cond
+              ((and dt
+                    (or (string-match "\\`\\* \\(.*?\\)\\(\\.\\)?\\'" dt)
+                        (string-match "\\`\\(.*(.*)\\)\\(\\.\\)?\\'" dt)))
+               ;; `dt' is already "complete".
+               (format "* %s." (match-string 1 dt)))
+              ((and dt (not (equal dt file)))
+               (format "* %s: (%s)." dt file))
+              (t (format "* %s." file)))))
+       (when dircat
         (concat "@dircategory " dircat "\n"
                 "@direntry\n"
                 (let ((dirdesc
-                       (let ((desc (plist-get info :texinfo-dirdesc)))
+                       (let ((desc (or (plist-get info :texinfo-dirdesc)
+                                       title)))
                          (cond ((not desc) nil)
                                ((string-suffix-p "." desc) desc)
                                (t (concat desc "."))))))
@@ -1590,7 +1582,7 @@ org-texinfo-planning
   (concat
    "@noindent"
    (mapconcat
-    'identity
+    #'identity
     (delq nil
          (list
           (let ((closed (org-element-property :closed planning)))