[Fix #1232] Switch to a simpler scheme for updating the modeline

Now it's done just once when a file is opened, which should be good
enough for everyone and as fast as possible.

It's funny that after so many years we simply came back to square one.
On the bright side - I can finally delete all the dreaded caching and
optimization code that was added mostly to address this.

projectile.el

@@ -3877,23 +3877,19 @@ is chosen."
 
 ;;; Projectile Minor mode
 (defcustom projectile-mode-line
-  '(:eval (format " Projectile[%s]"
-                  (projectile-project-name)))
-  "Mode line lighter for Projectile.
-
-The value of this variable is a mode line template as in
-`mode-line-format'.  See Info Node `(elisp)Mode Line Format' for
-details about mode line templates.
-
-Customize this variable to change how Projectile displays its
-status in the mode line.  The default value displays the project
-name.  Set this variable to nil to disable the mode line
-entirely."
+  "Projectile"
+  "Mode line lighter for Projectile."
   :group 'projectile
-  :type 'sexp
-  :risky t
+  :type 'string
   :package-version '(projectile . "0.12.0"))
 
+(defun projectile-update-mode-line ()
+  "Report project in mode-line."
+  (let* ((project-name (projectile-project-name))
+         (message (format " %s[%s]" projectile-mode-line project-name)))
+    (setq projectile-mode-line message))
+  (force-mode-line-update))
+
 (defvar projectile-command-map
   (let ((map (make-sparse-keymap)))
     (define-key map (kbd "4 a") #'projectile-find-other-file-other-window)
@@ -4055,11 +4051,13 @@ Otherwise behave as if called interactively.
     (add-hook 'find-file-hook 'projectile-find-file-hook-function)
     (add-hook 'projectile-find-dir-hook #'projectile-track-known-projects-find-file-hook t)
     (add-hook 'dired-before-readin-hook #'projectile-track-known-projects-find-file-hook t t)
+    (add-hook 'find-file-hook 'projectile-update-mode-line t t)
     (ad-activate 'compilation-find-file)
     (ad-activate 'delete-file))
    (t
     (remove-hook 'find-file-hook #'projectile-find-file-hook-function)
     (remove-hook 'dired-before-readin-hook #'projectile-track-known-projects-find-file-hook t)
+    (remove-hook 'find-file-hook #'projectile-update-mode-line)
     (ad-deactivate 'compilation-find-file)
     (ad-deactivate 'delete-file))))