sh: Do not enter consecutive duplicates into the history.

This simply sets a flag in libedit. It has a shortcoming in that it does not
apply to multi-line commands.

Note that a configuration option for this is not going to happen, but always
having this seems better than not having it. NetBSD has done the same.

PR:		bin/54683
Obtained from:	NetBSD
MFC after:	1 month

bin/sh/histedit.c

@@ -168,6 +168,7 @@ sethistsize(hs)
 		   (histsize = atoi(hs)) < 0)
 			histsize = 100;
 		history(hist, &he, H_SETSIZE, histsize);
+		history(hist, &he, H_SETUNIQUE, 1);
 	}
 }