Wrap some set-auto-mode calls with delay-mode-hooks (bug#39190)

* lisp/gnus/mm-view.el (mm-display-inline-fontify):
* lisp/vc/diff-mode.el (diff-syntax-fontify-props):
* lisp/vc/vc.el (vc-find-revision-no-save):
Add delay-mode-hooks around set-auto-mode calls
to not run hooks that might assume buffer-file-name
really associates buffer with a file.

Author: link0ff

lisp/gnus/mm-view.el

@@ -497,7 +497,9 @@ If MODE is not set, try to find mode automatically."
 	    (let ((auto-mode-alist
 		   (delq (rassq 'doc-view-mode-maybe auto-mode-alist)
 			 (copy-sequence auto-mode-alist))))
-	      (set-auto-mode)
+	      ;; Don't run hooks that might assume buffer-file-name
+	      ;; really associates buffer with a file (bug#39190).
+	      (delay-mode-hooks (set-auto-mode))
 	      (setq mode major-mode)))
 	  ;; Do not fontify if the guess mode is fundamental.
 	  (unless (eq major-mode 'fundamental-mode)

lisp/vc/diff-mode.el

@@ -2719,7 +2719,9 @@ hunk text is not found in the source file."
     (cl-assert (null buffer-file-name))
     (let ((enable-local-variables :safe) ;; to find `mode:'
           (buffer-file-name file))
-      (set-auto-mode)
+      ;; Don't run hooks that might assume buffer-file-name
+      ;; really associates buffer with a file (bug#39190).
+      (delay-mode-hooks (set-auto-mode))
       ;; FIXME: Is this really worth the trouble?
       (when (and (fboundp 'generic-mode-find-file-hook)
                  (memq #'generic-mode-find-file-hook

lisp/vc/vc.el

@@ -2098,7 +2098,9 @@ Unlike `vc-find-revision-save', doesn't save the buffer to the file."
                     ;; For non-interactive, skip any questions
                     (let ((enable-local-variables :safe) ;; to find `mode:'
                           (buffer-file-name file))
-                      (ignore-errors (set-auto-mode)))
+                      ;; Don't run hooks that might assume buffer-file-name
+                      ;; really associates buffer with a file (bug#39190).
+                      (ignore-errors (delay-mode-hooks (set-auto-mode))))
                   (normal-mode))
 	        (set-buffer-modified-p nil)
                 (setq buffer-read-only t))