patch 8.0.0548: saving the redo buffer only works one time

Problem:    Saving the redo buffer only works one time, resulting in the "."
            command not working well for a function call inside another
            function call. (Ingo Karkat)
Solution:   Save the redo buffer at every user function call. (closes #1619)

src/structs.h

@@ -515,6 +515,12 @@ struct buffheader
     int		bh_space;	/* space in bh_curr for appending */
 };
 
+typedef struct
+{
+    buffheader_T sr_redobuff;
+    buffheader_T sr_old_redobuff;
+} save_redo_T;
+
 /*
  * used for completion on the command line
  */