http: use hostname in credential description

Until now, a request for an http password looked like:

  Username:
  Password:

Now it will look like:

  Username for 'example.com':
  Password for 'example.com':

Picked-from: Jeff King <peff@peff.net>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5550-http-fetch.sh

@@ -51,8 +51,8 @@ test_expect_success 'setup askpass helpers' '
 	GIT_ASKPASS="$PWD/askpass" &&
 	export GIT_ASKPASS &&
 	>askpass-expect-none &&
-	echo "askpass: Password: " >askpass-expect-pass &&
-	{ echo "askpass: Username: " &&
+	echo "askpass: Password for '\''$HTTPD_DEST'\'': " >askpass-expect-pass &&
+	{ echo "askpass: Username for '\''$HTTPD_DEST'\'': " &&
 	  cat askpass-expect-pass
 	} >askpass-expect-both
 '