patch 8.1.0240: g:actual_curbuf set in wrong scope

Problem:    g:actual_curbuf set in wrong scope. (Daniel Hahler)
Solution:   Prepend the "g:" name space. (closes #3279)
diff --git a/src/buffer.c b/src/buffer.c
index f76d849..64ba5ab 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -4229,7 +4229,7 @@
 
 #ifdef FEAT_EVAL
 	    vim_snprintf((char *)tmp, sizeof(tmp), "%d", curbuf->b_fnum);
-	    set_internal_string_var((char_u *)"actual_curbuf", tmp);
+	    set_internal_string_var((char_u *)"g:actual_curbuf", tmp);
 
 	    save_curbuf = curbuf;
 	    save_curwin = curwin;