git-version.py: proper support for tags

git-version.py: proper support for tags

Git magic to get branch name

  git rev-parse --symbolic-full-name HEAD

doesn't work when we're on tag, which is the case when
we build releases.

First, try to get tag name with

   git describe --exact-match

and if this fails, get branch name as before.

Use subprocess.Popen() to suppress stdout/stderr output.

Github: Fixes OpenVPN/openvpn#199

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20221219155638.497-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25773.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 91ab3d022e2652a46e6d6f25ab62f7c903e583c1)