dialogs: indicate when GIMP is up to date in About dialog

To make more clear to users that GIMP is up to date, the About dialog now
shows "Up to date" along with the timestamp of the last update check.
If there's a new update available, it will show only the timestamp
of the last update check.

Author: GabryBarbe

app/dialogs/about-dialog.c

@@ -510,11 +510,20 @@ about_dialog_add_update (GimpAboutDialog *dialog,
       datetime = g_date_time_new_from_unix_local (config->check_update_timestamp);
       date = g_date_time_format (datetime, "%x");
       time = g_date_time_format (datetime, "%X");
-      /* Translators: first string is the date in the locale's date
-       * representation (e.g., 12/31/99), second is the time in the
-       * locale's time representation (e.g., 23:13:48).
-       */
-      subtext = g_strdup_printf (_("Last checked on %s at %s"), date, time);
+
+      if (config->last_known_release != NULL)
+        /* Translators: first string is the date in the locale's date
+        * representation (e.g., 12/31/99), second is the time in the
+        * locale's time representation (e.g., 23:13:48).
+        */
+        subtext = g_strdup_printf (_("Last checked on %s at %s"), date, time);
+      else
+        /* Translators: first string is the date in the locale's date
+        * representation (e.g., 12/31/99), second is the time in the
+        * locale's time representation (e.g., 23:13:48).
+        */
+        subtext = g_strdup_printf (_("Up to date as of %s at %s"), date, time);
+
       g_date_time_unref (datetime);
       g_free (date);
       g_free (time);