shada: Run set_last_cursor before writing shada file

runtime/doc/motion.txt

@@ -877,7 +877,8 @@ was made yet in the current file.
 			for each opened file.
 			Only one position is remembered per buffer, not one
 			for each window.  As long as the buffer is visible in
-			a window the position won't be changed.
+			a window the position won't be changed.  Mark is also 
+			reset when |:wshada| is run.
 
 							*'^* *`^*
 '^  `^			To the position where the cursor was the last time

runtime/doc/starting.txt

@@ -1169,6 +1169,8 @@ running) you have additional options:
 			cannot write ShaDa file!" check that no old temp files 
 			were left behind (e.g. ~/.nvim/shada/main.shada.tmp*).
 
+			Note: Executing :wshada will reset all |'quote| marks.
+
 						   *:wv* *:wviminfo*
 :wv[iminfo][!] [file]	Deprecated alias to |:wshada| command.
 

src/nvim/shada.c

@@ -2433,6 +2433,15 @@ static ShaDaWriteResult shada_write(ShaDaWriteDef *const sd_writer,
   msgpack_packer *const packer = msgpack_packer_new(sd_writer,
                                                     &msgpack_sd_writer_write);
 
+  // Set b_last_cursor for all the buffers that have a window.
+  //
+  // It is needed to correctly save '"' mark on exit. Has a side effect of
+  // setting '"' mark in all windows on :wshada to the current cursor
+  // position (basically what :wviminfo used to do).
+  FOR_ALL_TAB_WINDOWS(tp, wp) {
+    set_last_cursor(wp);
+  }
+
   FOR_ALL_BUFFERS(buf) {
     if (buf->b_ffname != NULL && shada_removable((char *) buf->b_ffname)) {
       int kh_ret;