patch 8.1.1218: cannot set a directory for a tab page

Problem:    Cannot set a directory for a tab page.
Solution:   Add the tab-local directory. (Yegappan Lakshmanan, closes #4212)
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index ac354cd..27029ba 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1479,6 +1479,12 @@
 EX(CMD_tabs,		"tabs",		ex_tabs,
 			TRLBAR|CMDWIN,
 			ADDR_TABS),
+EX(CMD_tcd,		"tcd",		ex_cd,
+			BANG|FILE1|TRLBAR|CMDWIN,
+			ADDR_OTHER),
+EX(CMD_tchdir,		"tchdir",	ex_cd,
+			BANG|FILE1|TRLBAR|CMDWIN,
+			ADDR_OTHER),
 EX(CMD_tcl,		"tcl",		ex_tcl,
 			RANGE|EXTRA|NEEDARG|CMDWIN|RESTRICT,
 			ADDR_LINES),