patch 8.1.0706: tabline is not always redrawn

Problem:    Tabline is not always redrawn when something that is used in
            'tabline' changes.
Solution:   Add ":redrawtabline" so that a plugin can at least cause the
            redraw when needed.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index aad8182..354cc11 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -296,6 +296,7 @@
 static void	ex_later(exarg_T *eap);
 static void	ex_redir(exarg_T *eap);
 static void	ex_redrawstatus(exarg_T *eap);
+static void	ex_redrawtabline(exarg_T *eap);
 static void	close_redir(void);
 static void	ex_mkrc(exarg_T *eap);
 static void	ex_mark(exarg_T *eap);
@@ -9918,6 +9919,25 @@
     out_flush();
 }
 
+/*
+ * ":redrawtabline": force redraw of the tabline
+ */
+    static void
+ex_redrawtabline(exarg_T *eap UNUSED)
+{
+    int		r = RedrawingDisabled;
+    int		p = p_lz;
+
+    RedrawingDisabled = 0;
+    p_lz = FALSE;
+
+    draw_tabline();
+
+    RedrawingDisabled = r;
+    p_lz = p;
+    out_flush();
+}
+
     static void
 close_redir(void)
 {