Autosave history fixes

If autosaving history we keep track of last savetime immediately.

Also do improved logs for times spent on writing history and writing the xmp file
and always do a log when autosaving has been disabled.

src/develop/develop.c

@@ -714,34 +714,40 @@ static void _dev_auto_save(dt_develop_t *dev)
   static double last = 0.0;
 
   const double user_delay = (double)dt_conf_get_int("autosave_interval");
-  const double now = dt_get_wtime();
-
   const dt_imgid_t imgid = dev->image_storage.id;
 
   /* We can only autosave database & xmp while we have a valid image id
      and we are not currently loading or changing it in main darkroom
   */
   const gboolean saving = (user_delay >= 1.0)
-                        && ((now - last) > user_delay)
+                        && ((dt_get_wtime() - last) > user_delay)
                         && !dev->full.pipe->loading
                         && dev->requested_id == imgid
                         && dt_is_valid_imgid(imgid);
 
   if(saving)
   {
+    // avoid saving for fast repeated calls
+    last = dt_get_wtime();
     // Ok, lets save status for image
     dt_dev_write_history(dev);
+
+    // check for slow drives
+    const double start = dt_get_wtime();
     dt_image_write_sidecar_file(imgid);
-    last = now;
+    const double after = dt_get_wtime();
 
-    const double spent = dt_get_wtime() - now;
-    dt_print(DT_DEBUG_DEV, "autosave history took %fsec\n", spent);
+    dt_print(DT_DEBUG_DEV, "autosave history took %.3fs (hist) %.3fs (drive)\n",
+        start - last, after - start);
 
     // if writing to database and the xmp took too long we disable
     // autosaving mode for this session
-    if(spent > 0.5)
+    if((after - last) > 0.5)
     {
       dev->autosaving = FALSE;
+      dt_print(DT_DEBUG_ALL, "autosave history disabled, took %.3fs (hist) %.3fs (drive)\n",
+        start - last, after - start);
+
       dt_control_log(_("autosaving history has been disabled"
                        " for this session because of a slow drive used"));
     }