clone: Add an option to set up a mirror

The command line

	$ git clone --mirror $URL

is now a short-hand for

	$ git clone --bare $URL
	$ (cd $(basename $URL) && git remote add --mirror origin $URL)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-remote.c

@@ -115,7 +115,7 @@ static int add(int argc, const char **argv)
 	if (mirror) {
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "remote.%s.mirror", name);
-		if (git_config_set(buf.buf, "yes"))
+		if (git_config_set(buf.buf, "true"))
 			return 1;
 	}