please.sh: prefix the announcement mail with [ANNOUNCE]

To make filtering mailboxes much easier, let's just prefix the
announcement mail with the same prefix as Junio prefixes the
announcements of upstream Git.

Desired-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

please.sh

@@ -753,7 +753,7 @@ publish () { #
 		"From: $(git var GIT_COMMITTER_IDENT | sed -e 's/>.*/>/')" \
 		"Date: $(date -R)" \
 		"To: git-for-windows@googlegroups.com, git@vger.kernel.org" \
-		"Subject: Announcing Git for Windows $displayver" \
+		"Subject: [ANNOUNCE] Git for Windows $displayver" \
 		"$prefix" \
 		"$rendered" \
 		"$checksums" \