vim-patch:8.1.1510: a plugin cannot easily expand a command like done internally

Problem:    A plugin cannot easily expand a command like done internally.
Solution:   Add the expandcmd() function. (Yegappan Lakshmanan, closes vim/vim#4514)
80dad48c50

src/nvim/eval.lua

@@ -102,6 +102,7 @@ return {
     exists={args=1},
     exp={args=1, func="float_op_wrapper", data="&exp"},
     expand={args={1, 3}},
+    expandcmd={args=1},
     extend={args={2, 3}},
     feedkeys={args={1, 2}},
     file_readable={args=1, func='f_filereadable'},  -- obsolete

src/nvim/eval/funcs.c

@@ -2103,6 +2103,31 @@ static void f_menu_get(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   menu_get((char_u *)tv_get_string(&argvars[0]), modes, rettv->vval.v_list);
 }
 
+// "expandcmd()" function
+// Expand all the special characters in a command string.
+static void f_expandcmd(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  char_u *errormsg = NULL;
+
+  rettv->v_type = VAR_STRING;
+  char_u *cmdstr = (char_u *)xstrdup(tv_get_string(&argvars[0]));
+
+  exarg_T eap = {
+    .cmd = cmdstr,
+    .arg = cmdstr,
+    .usefilter = false,
+    .nextcmd = NULL,
+    .cmdidx = CMD_USER,
+  };
+  eap.argt |= NOSPC;
+
+  expand_filename(&eap, &cmdstr, &errormsg);
+  if (errormsg != NULL && *errormsg != NUL) {
+    EMSG(errormsg);
+  }
+  rettv->vval.v_string = cmdstr;
+}
+
 /*
  * "extend(list, list [, idx])" function
  * "extend(dict, dict [, action])" function