patch 8.2.3638: getcompletion() always passes zero as position

Problem:    getcompletion() always passes zero as position to custom
            completion function.
Solution:   Pass the pattern length. (closes #9173)

src/cmdexpand.c

@@ -2900,6 +2900,7 @@ f_getcompletion(typval_T *argvars, typval_T *rettv)
 		|| check_for_opt_bool_arg(argvars, 2) == FAIL))
 	return;
 
+    pat = tv_get_string(&argvars[0]);
     if (argvars[1].v_type != VAR_STRING)
     {
 	semsg(_(e_invarg2), "type must be a string");
@@ -2920,12 +2921,13 @@ f_getcompletion(typval_T *argvars, typval_T *rettv)
     ExpandInit(&xpc);
     if (STRCMP(type, "cmdline") == 0)
     {
-	set_one_cmd_context(&xpc, tv_get_string(&argvars[0]));
+	set_one_cmd_context(&xpc, pat);
 	xpc.xp_pattern_len = (int)STRLEN(xpc.xp_pattern);
+	xpc.xp_col = (int)STRLEN(pat);
     }
     else
     {
-	xpc.xp_pattern = tv_get_string(&argvars[0]);
+	xpc.xp_pattern = pat;
 	xpc.xp_pattern_len = (int)STRLEN(xpc.xp_pattern);
 
 	xpc.xp_context = cmdcomplete_str_to_type(type);

src/testdir/test_cmdline.vim

@@ -424,14 +424,17 @@ func Test_getcompletion()
   call assert_equal([], l)
 
   func T(a, c, p)
+    let g:cmdline_compl_params = [a:a, a:c, a:p]
     return "oneA\noneB\noneC"
   endfunc
   command -nargs=1 -complete=custom,T MyCmd
   let l = getcompletion('MyCmd ', 'cmdline')
   call assert_equal(['oneA', 'oneB', 'oneC'], l)
+  call assert_equal(['', 'MyCmd ', 6], g:cmdline_compl_params)
 
   delcommand MyCmd
   delfunc T
+  unlet g:cmdline_compl_params
 
   " For others test if the name is recognized.
   let names = ['buffer', 'environment', 'file_in_path', 'mapping', 'tag', 'tag_listfiles', 'user']

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3638,
 /**/
     3637,
 /**/