Speed up smerge-refine-regions by avoiding fsync

* lisp/vc/smerge-mode.el (smerge-refine-regions): Bind
write-region-inhibit-fsync to t.  This was reported in
https://github.com/magit/magit/pull/2834 to give a noticable speedup.

lisp/vc/smerge-mode.el

@@ -1075,9 +1075,10 @@ used to replace chars to try and eliminate some spurious differences."
           (if smerge-refine-weight-hack (make-hash-table :test #'equal))))
     (unless (markerp beg1) (setq beg1 (copy-marker beg1)))
     (unless (markerp beg2) (setq beg2 (copy-marker beg2)))
-    ;; Chop up regions into smaller elements and save into files.
-    (smerge--refine-chopup-region beg1 end1 file1 preproc)
-    (smerge--refine-chopup-region beg2 end2 file2 preproc)
+    (let ((write-region-inhibit-fsync t)) ; Don't fsync temp files (Bug#12747).
+      ;; Chop up regions into smaller elements and save into files.
+      (smerge--refine-chopup-region beg1 end1 file1 preproc)
+      (smerge--refine-chopup-region beg2 end2 file2 preproc))
 
     ;; Call diff on those files.
     (unwind-protect