patch 8.2.1967: the session file does not restore the alternate file

Problem:    The session file does not restore the alternate file.
Solution:   Add ":balt".  Works like ":badd" and also sets the buffer as the
            alternate file.  Use it in the session file. (closes #7269,
            closes #6714)
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index d53b6df..4fe62ca 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -178,6 +178,9 @@
 EXCMD(CMD_badd,		"badd",		ex_edit,
 	EX_NEEDARG|EX_FILE1|EX_CMDARG|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
+EXCMD(CMD_balt,		"balt",		ex_edit,
+	EX_NEEDARG|EX_FILE1|EX_CMDARG|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
+	ADDR_NONE),
 EXCMD(CMD_bdelete,	"bdelete",	ex_bunload,
 	EX_BANG|EX_RANGE|EX_BUFNAME|EX_COUNT|EX_EXTRA|EX_TRLBAR,
 	ADDR_BUFFERS),