vim-patch:8.0.0548: saving the redo buffer only works one time (#8629)

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 vim/vim#1619)
d4863aa99e

src/nvim/buffer_defs.h

@@ -143,6 +143,12 @@ struct buffheader {
   size_t bh_space;          // space in bh_curr for appending
 };
 
+typedef struct
+{
+  buffheader_T sr_redobuff;
+  buffheader_T sr_old_redobuff;
+} save_redo_T;
+
 /*
  * Structure that contains all options that are local to a window.
  * Used twice in a window: for the current buffer and for all buffers.