patch 8.2.1380: Vim9: return type of getreg() is always a string

Problem:    Vim9: return type of getreg() is always a string.
Solution:   Use list of strings when there are three arguments. (closes #6633)

src/evalfunc.c

@@ -393,6 +393,15 @@ ret_remove(int argcount UNUSED, type_T **argtypes)
     return &t_any;
 }
 
+    static type_T *
+ret_getreg(int argcount, type_T **argtypes UNUSED)
+{
+    // Assume that if the third argument is passed it's non-zero
+    if (argcount == 3)
+	return &t_list_string;
+    return &t_string;
+}
+
 static type_T *ret_f_function(int argcount, type_T **argtypes);
 
 /*
@@ -641,7 +650,7 @@ static funcentry_T global_functions[] =
     {"getpid",		0, 0, 0,	  ret_number,	f_getpid},
     {"getpos",		1, 1, FEARG_1,	  ret_list_number,	f_getpos},
     {"getqflist",	0, 1, 0,	  ret_list_or_dict_0,	f_getqflist},
-    {"getreg",		0, 3, FEARG_1,	  ret_string,	f_getreg},
+    {"getreg",		0, 3, FEARG_1,	  ret_getreg,	f_getreg},
     {"getreginfo",	0, 1, FEARG_1,	  ret_dict_any,	f_getreginfo},
     {"getregtype",	0, 1, FEARG_1,	  ret_string,	f_getregtype},
     {"gettabinfo",	0, 1, FEARG_1,	  ret_list_dict_any,	f_gettabinfo},

src/testdir/test_vim9_func.vim

@@ -1202,6 +1202,12 @@ def Test_filter_return_type()
   assert_equal(6, res)
 enddef
 
+def Test_getreg_return_type()
+  let s1: string = getreg('"')
+  let s2: string = getreg('"', 1)
+  let s3: list<string> = getreg('"', 1, 1)
+enddef
+
 def Wrong_dict_key_type(items: list<number>): list<number>
   return filter(items, {_, val -> get({val: 1}, 'x')})
 enddef

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1380,
 /**/
     1379,
 /**/