patch 8.0.0300: cannot stop diffing hidden buffers

Problem:    Cannot stop diffing hidden buffers. (Daniel Hahler)
Solution:   When using :diffoff! make the whole list if diffed buffers empty.
            (closes #736)
diff --git a/src/diff.c b/src/diff.c
index a4a2acb..8d7421d 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -139,6 +139,23 @@
 }
 
 /*
+ * Remove all buffers to make diffs for.
+ */
+    static void
+diff_buf_clear(void)
+{
+    int		i;
+
+    for (i = 0; i < DB_COUNT; ++i)
+	if (curtab->tp_diffbuf[i] != NULL)
+	{
+	    curtab->tp_diffbuf[i] = NULL;
+	    curtab->tp_diff_invalid = TRUE;
+	    diff_redraw(TRUE);
+	}
+}
+
+/*
  * Find buffer "buf" in the list of diff buffers for the current tab page.
  * Return its index or DB_COUNT if not found.
  */
@@ -1257,6 +1274,10 @@
 #endif
     }
 
+    /* Also remove hidden buffers from the list. */
+    if (eap->forceit)
+	diff_buf_clear();
+
 #ifdef FEAT_SCROLLBIND
     /* Remove "hor" from from 'scrollopt' if there are no diff windows left. */
     if (!diffwin && vim_strchr(p_sbo, 'h') != NULL)