feat(eval/method): partially port v8.1.1925

Adds method call support for all functions in the patch, but it cannot
be fully ported due to missing tests for:

- getcwd(): requires chdir() and Test_chdir_func() from v8.1.1291.

Note that the method call tests for getreg() and getregtype() were
removed in v8.2.1547, which has already been ported, but doesn't seem to
have been replaced with a new test...

This patch also makes getchangelist()'s argument optional (defaults to
the current buffer).

eval.txt includes a typo for gettabwinvar(), which is fixed in
v8.1.1952.

src/nvim/eval/funcs.c

@@ -3088,10 +3088,16 @@ f_getbufvar_end:
 static void f_getchangelist(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   tv_list_alloc_ret(rettv, 2);
-  vim_ignored = tv_get_number(&argvars[0]);  // issue errmsg if type error
-  emsg_off++;
-  const buf_T *const buf = tv_get_buf(&argvars[0], false);
-  emsg_off--;
+
+  const buf_T *buf;
+  if (argvars[0].v_type == VAR_UNKNOWN) {
+    buf = curbuf;
+  } else {
+    vim_ignored = tv_get_number(&argvars[0]);  // issue errmsg if type error
+    emsg_off++;
+    buf = tv_get_buf(&argvars[0], false);
+    emsg_off--;
+  }
   if (buf == NULL) {
     return;
   }