patch 7.4.1658

Problem:    A plugin does not know when VimEnter autocommands were already
            triggered.
Solution:   Add the v:vim_did_enter variable.

src/eval.c

@@ -372,6 +372,7 @@ static struct vimvar
     {VV_NAME("true",		 VAR_SPECIAL), VV_RO},
     {VV_NAME("null",		 VAR_SPECIAL), VV_RO},
     {VV_NAME("none",		 VAR_SPECIAL), VV_RO},
+    {VV_NAME("vim_did_enter",	 VAR_NUMBER), VV_RO},
 };
 
 /* shorthand */