patch 8.1.1049: when user tries to exit with CTRL-C message is confusing

Problem:    When user tries to exit with CTRL-C message is confusing.
Solution:   Only mention ":qa!" when there is a changed buffer. (closes #4163)

src/undo.c

@@ -3530,6 +3530,19 @@ bufIsChanged(buf_T *buf)
     return bufIsChangedNotTerm(buf);
 }
 
+/*
+ * Return TRUE if any buffer has changes.  Also buffers that are not written.
+ */
+    int
+anyBufIsChanged(void)
+{
+    buf_T *buf;
+
+    FOR_ALL_BUFFERS(buf)
+	if (bufIsChanged(buf))
+	    return TRUE;
+}
+
 /*
  * Like bufIsChanged() but ignoring a terminal window.
  */