patch 8.2.1413: previous tab page not usable from an Ex command

Problem:    Previous tab page not usable from an Ex command.
Solution:   Add the "#" argument for :tabnext et al. (Yegappan Lakshmanan,
            closes #6677)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 96eac7f..502f33e 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -5412,6 +5412,15 @@
 	{
 	    if (STRCMP(p, "$") == 0)
 		tab_number = LAST_TAB_NR;
+	    else if (STRCMP(p, "#") == 0)
+		if (valid_tabpage(lastused_tabpage))
+		    tab_number = tabpage_index(lastused_tabpage);
+		else
+		{
+		    eap->errmsg = ex_errmsg(e_invargval, eap->arg);
+		    tab_number = 0;
+		    goto theend;
+		}
 	    else if (p == p_save || *p_save == '-' || *p != NUL
 		    || tab_number > LAST_TAB_NR)
 	    {