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_cmds.h b/src/ex_cmds.h
index 49e7fc7..6bb930c 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1175,6 +1175,9 @@
 EX(CMD_redrawstatus,	"redrawstatus",	ex_redrawstatus,
 			BANG|TRLBAR|CMDWIN,
 			ADDR_LINES),
+EX(CMD_redrawtabline,	"redrawtabline", ex_redrawtabline,
+			TRLBAR|CMDWIN,
+			ADDR_LINES),
 EX(CMD_registers,	"registers",	ex_display,
 			EXTRA|NOTRLCOM|TRLBAR|CMDWIN,
 			ADDR_LINES),