dev-tools/gerrit-send-mail.py: include Gerrit URL into the commit message

dev-tools/gerrit-send-mail.py: include Gerrit URL into the commit message

We used to reference to one of the mailing list archives as the formal
reference for the patch and the review discussions.  With the problematic
reliability of the archives, I've started to manually add the URLs in
the OpenVPN Gerrit to the commits because they give a better reference
and (usually) all the feedback is there in a useful format.

So, enhance this dev-tool script to do this automatically.

Change-Id: Idb137ecbade4b0584a4d74aee34978062e247bc5
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1196
Message-Id: <20250917120720.30738-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59234672/
Signed-off-by: Gert Doering <gert@greenie.muc.de>