diff-hl-flydiff-update: Update diff-hl-flydiff-modified-tick here

To account for the case when the condition in it succeeds, but the buffer is
up-to-date and thus not ...-modifier-p, so diff-hl-changes-buffer was never
called - but diff-hl--resolve's callback did.

Thanks to @ermingol23 for reporting: #278

Author: dgutov

diff-hl-flydiff.el

@@ -42,7 +42,6 @@
 
 (defun diff-hl-flydiff-changes-buffer (file backend &optional new-rev buffer)
   (setq buffer (or buffer " *diff-hl-diff*"))
-  (setq diff-hl-flydiff-modified-tick (buffer-chars-modified-tick))
   (if new-rev
       (diff-hl-with-diff-switches
        (diff-hl-diff-against-reference file backend buffer new-rev))
@@ -56,6 +55,7 @@
              (or (not file)
                  (file-remote-p default-directory)
                  (not (file-exists-p file)))))
+    (setq diff-hl-flydiff-modified-tick (buffer-chars-modified-tick))
     (diff-hl-update)))
 
 (defun diff-hl-flydiff/modified-p (state)