patch 8.2.2468: not easy to get the full command name from a shortened one

Problem:    Not easy to get the full command name from a shortened one.
Solution:   Add fullcommand(). (Martin Tournoij, closes #7777)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index d31b56c..da499c9 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -3668,6 +3668,33 @@
 	return 0;	// trailing garbage
     return (ea.cmdidx == CMD_SIZE ? 0 : (full ? 2 : 1));
 }
+
+/*
+ * "fullcommand" function
+ */
+    void
+f_fullcommand(typval_T *argvars, typval_T *rettv)
+{
+    exarg_T  ea;
+    char_u   *name = argvars[0].vval.v_string;
+    char_u   *p;
+
+    while (name[0] != NUL && name[0] == ':')
+	name++;
+    name = skip_range(name, TRUE, NULL);
+
+    rettv->v_type = VAR_STRING;
+
+    ea.cmd = (*name == '2' || *name == '3') ? name + 1 : name;
+    ea.cmdidx = (cmdidx_T)0;
+    p = find_ex_command(&ea, NULL, NULL, NULL);
+    if (p == NULL || ea.cmdidx == CMD_SIZE)
+	return;
+
+    rettv->vval.v_string = vim_strsave(IS_USER_CMDIDX(ea.cmdidx)
+				    ? get_user_commands(NULL, ea.useridx)
+				    : cmdnames[ea.cmdidx].cmd_name);
+}
 #endif
 
     cmdidx_T