Support display-sort-function in completion-category-overrides (bug#68214)

* doc/lispref/minibuf.texi (Completion Variables):
Add 'display-sort-function' to the table of
'completion-category-overrides'.

* lisp/calendar/calendar.el (calendar-read-date): Add metadata
category 'calendar-month' for completing-read reading a month name.

* lisp/minibuffer.el (completion-category-defaults):
Add 'display-sort-function' with identity for the category 'calendar-month'.
(completion-category-overrides): Add customization for completion sorting
with 'display-sort-function' and a choice like in 'completions-sort'.
(completion-metadata-override-get): New function.
(minibuffer-completion-help): Use 'completion-metadata-override-get'
instead of 'completion-metadata-get' to get sort-fun from
'display-sort-function'.

doc/lispref/minibuf.texi

@@ -1878,6 +1878,12 @@ The value should be a list of completion styles (symbols).
 The value should be a value for @code{completion-cycle-threshold}
 (@pxref{Completion Options,,, emacs, The GNU Emacs Manual}) for this
 category.
+
+@item display-sort-function
+The possible values are: @code{nil} that means to use either the sorting
+function from metadata or if it's nil then fall back to @code{completions-sort};
+@code{identity} that means to not use any sorting to keep the original order;
+and other values are the same as in @code{completions-sort}.
 @end table
 
 @noindent

etc/NEWS

@@ -740,6 +740,12 @@ When 'completions-sort' is set to 'historical', completion candidates
 will be first sorted alphabetically, and then re-sorted by their order
 in the minibuffer history, with more recent candidates appearing first.
 
++++
+*** 'completion-category-overrides' supports 'display-sort-function'.
+You can now customize the sorting order for any category in
+'completion-category-overrides' that will override the sorting order
+defined in the metadata or in 'completions-sort'.
+
 ** Pcomplete
 
 ---

lisp/calendar/calendar.el

@@ -2339,7 +2339,11 @@ returned is (month year)."
          (month (cdr (assoc-string
                       (completing-read
                        (format-prompt "Month name" defmon)
-                       (append month-array nil)
+                       (lambda (string pred action)
+	                 (if (eq action 'metadata)
+		             '(metadata (category . calendar-month))
+	                   (complete-with-action
+                            action (append month-array nil) string pred)))
                        nil t nil nil defmon)
                       (calendar-make-alist month-array 1) t)))
          (defday (calendar-extract-day default-date))

lisp/minibuffer.el

@@ -1135,12 +1135,14 @@ styles for specific categories, such as files, buffers, etc."
     (project-file (styles . (substring)))
     (xref-location (styles . (substring)))
     (info-menu (styles . (basic substring)))
-    (symbol-help (styles . (basic shorthand substring))))
+    (symbol-help (styles . (basic shorthand substring)))
+    (calendar-month (display-sort-function . identity)))
   "Default settings for specific completion categories.
 Each entry has the shape (CATEGORY . ALIST) where ALIST is
 an association list that can specify properties such as:
 - `styles': the list of `completion-styles' to use for that category.
 - `cycle': the `completion-cycle-threshold' to use for that category.
+- `display-sort-function': the sorting function.
 Categories are symbols such as `buffer' and `file', used when
 completing buffer and file names, respectively.
 
@@ -1148,10 +1150,16 @@ Also see `completion-category-overrides'.")
 
 (defcustom completion-category-overrides nil
   "List of category-specific user overrides for completion styles.
+
 Each override has the shape (CATEGORY . ALIST) where ALIST is
 an association list that can specify properties such as:
 - `styles': the list of `completion-styles' to use for that category.
 - `cycle': the `completion-cycle-threshold' to use for that category.
+- `display-sort-function': where `nil' means to use either the sorting
+function from metadata or if it's nil then fall back to `completions-sort';
+`identity' means to not use any sorting to keep the original order;
+and other values are the same as in `completions-sort'.
+
 Categories are symbols such as `buffer' and `file', used when
 completing buffer and file names, respectively.
 
@@ -1171,12 +1179,28 @@ overrides the default specified in `completion-category-defaults'."
 		 ,completion--styles-type)
            (cons :tag "Completion Cycling"
 		 (const :tag "Select one value from the menu." cycle)
-                 ,completion--cycling-threshold-type))))
+                 ,completion--cycling-threshold-type)
+           (cons :tag "Completion Sorting"
+                 (const :tag "Select one value from the menu."
+                        display-sort-function)
+                 (choice (const :tag "Use default" nil)
+                         (const :tag "No sorting" identity)
+                         (const :tag "Alphabetical sorting"
+                                minibuffer-sort-alphabetically)
+                         (const :tag "Historical sorting"
+                                minibuffer-sort-by-history)
+                         (function :tag "Custom function"))))))
 
 (defun completion--category-override (category tag)
   (or (assq tag (cdr (assq category completion-category-overrides)))
       (assq tag (cdr (assq category completion-category-defaults)))))
 
+(defun completion-metadata-override-get (metadata prop)
+  (if-let ((cat (completion-metadata-get metadata 'category))
+           (over (completion--category-override cat prop)))
+      (cdr over)
+    (completion-metadata-get metadata prop)))
+
 (defun completion--styles (metadata)
   (let* ((cat (completion-metadata-get metadata 'category))
          (over (completion--category-override cat 'styles)))
@@ -2522,7 +2546,7 @@ The candidate will still be chosen by `choose-completion' unless
              (aff-fun (or (completion-metadata-get all-md 'affixation-function)
                           (plist-get completion-extra-properties
                                      :affixation-function)))
-             (sort-fun (completion-metadata-get all-md 'display-sort-function))
+             (sort-fun (completion-metadata-override-get all-md 'display-sort-function))
              (group-fun (completion-metadata-get all-md 'group-function))
              (mainbuf (current-buffer))
              ;; If the *Completions* buffer is shown in a new