Consolidate 'widen' calls

* lisp/progmodes/prog-mode.el (prog-indentation-context):
Un-document all elements but the first.
(prog-widen): Remove.
(http://lists.gnu.org/archive/html/emacs-devel/2017-12/msg00321.html)

* doc/lispref/text.texi (Mode-Specific Indent): Update.

* lisp/progmodes/ruby-mode.el (ruby-calculate-indent):
Don't call widen.

* lisp/progmodes/python.el
(python-indent-guess-indent-offset)
(python-info-current-defun): Replace prog-widen with widen;
these functions are not called during indentation.
(python-indent-context)
(python-indent--calculate-indentation)
(python-info-dedenter-opening-block-message)
(python-info-line-ends-backslash-p)
(python-info-beginning-of-backslash)
(python-info-continuation-line-p)
(python-info-current-defun): Remove 'widen' calls.

* lisp/indent.el (indent-according-to-mode)
(indent-for-tab-command, indent-region): Move them here.

* lisp/textmodes/mhtml-mode.el (mhtml-indent-line):
Bind prog-indentation-context to one-element list.

lisp/progmodes/python.el

@@ -287,10 +287,6 @@
 ;;; 24.x Compat
 
 
-(unless (fboundp 'prog-widen)
-  (defun prog-widen ()
-    (widen)))
-
 (unless (fboundp 'prog-first-column)
   (defun prog-first-column ()
     0))
@@ -785,7 +781,7 @@ work on `python-indent-calculate-indentation' instead."
   (interactive)
   (save-excursion
     (save-restriction
-      (prog-widen)
+      (widen)
       (goto-char (point-min))
       (let ((block-end))
         (while (and (not block-end)
@@ -883,8 +879,6 @@ keyword
 :at-dedenter-block-start
  - Point is on a line starting a dedenter block.
  - START is the position where the dedenter block starts."
-  (save-restriction
-    (prog-widen)
     (let ((ppss (save-excursion
                   (beginning-of-line)
                   (syntax-ppss))))
@@ -1022,7 +1016,7 @@ keyword
                       (looking-at (python-rx block-ender)))
                     :after-block-end)
                    (t :after-line))
-             (point)))))))))
+             (point))))))))
 
 (defun python-indent--calculate-indentation ()
   "Internal implementation of `python-indent-calculate-indentation'.
@@ -1030,8 +1024,6 @@ May return an integer for the maximum possible indentation at
 current context or a list of integers.  The latter case is only
 happening for :at-dedenter-block-start context since the
 possibilities can be narrowed to specific indentation points."
-  (save-restriction
-    (prog-widen)
     (save-excursion
       (pcase (python-indent-context)
         (`(:no-indent . ,_) (prog-first-column)) ; usually 0
@@ -1081,7 +1073,7 @@ possibilities can be narrowed to specific indentation points."
         (`(,(or :inside-paren-newline-start-from-block) . ,start)
          ;; Add two indentation levels to make the suite stand out.
          (goto-char start)
-         (+ (current-indentation) (* python-indent-offset 2)))))))
+         (+ (current-indentation) (* python-indent-offset 2))))))
 
 (defun python-indent--calculate-levels (indentation)
   "Calculate levels list given INDENTATION.
@@ -4590,7 +4582,7 @@ Optional argument INCLUDE-TYPE indicates to include the type of the defun.
 This function can be used as the value of `add-log-current-defun-function'
 since it returns nil if point is not inside a defun."
   (save-restriction
-    (prog-widen)
+    (widen)
     (save-excursion
       (end-of-line 1)
       (let ((names)
@@ -4788,12 +4780,10 @@ likely an invalid python file."
   "Message the first line of the block the current statement closes."
   (let ((point (python-info-dedenter-opening-block-position)))
     (when point
-      (save-restriction
-        (prog-widen)
         (message "Closes %s" (save-excursion
                                (goto-char point)
                                (buffer-substring
-                                (point) (line-end-position))))))))
+                                (point) (line-end-position)))))))
 
 (defun python-info-dedenter-statement-p ()
   "Return point if current statement is a dedenter.
@@ -4809,8 +4799,6 @@ statement."
   "Return non-nil if current line ends with backslash.
 With optional argument LINE-NUMBER, check that line instead."
   (save-excursion
-    (save-restriction
-      (prog-widen)
       (when line-number
         (python-util-goto-line line-number))
       (while (and (not (eobp))
@@ -4819,14 +4807,12 @@ With optional argument LINE-NUMBER, check that line instead."
                   (not (equal (char-before (point)) ?\\)))
         (forward-line 1))
       (when (equal (char-before) ?\\)
-        (point-marker)))))
+        (point-marker))))
 
 (defun python-info-beginning-of-backslash (&optional line-number)
   "Return the point where the backslashed line start.
 Optional argument LINE-NUMBER forces the line number to check against."
   (save-excursion
-    (save-restriction
-      (prog-widen)
       (when line-number
         (python-util-goto-line line-number))
       (when (python-info-line-ends-backslash-p)
@@ -4835,15 +4821,13 @@ Optional argument LINE-NUMBER forces the line number to check against."
                  (python-syntax-context 'paren))
           (forward-line -1))
         (back-to-indentation)
-        (point-marker)))))
+        (point-marker))))
 
 (defun python-info-continuation-line-p ()
   "Check if current line is continuation of another.
 When current line is continuation of another return the point
 where the continued line ends."
   (save-excursion
-    (save-restriction
-      (prog-widen)
       (let* ((context-type (progn
                              (back-to-indentation)
                              (python-syntax-context-type)))
@@ -4869,7 +4853,7 @@ where the continued line ends."
                (python-util-forward-comment -1)
                (when (and (equal (1- line-start) (line-number-at-pos))
                           (python-info-line-ends-backslash-p))
-                 (point-marker))))))))
+                 (point-marker)))))))
 
 (defun python-info-block-continuation-line-p ()
   "Return non-nil if current line is a continuation of a block."