git-gui provides an implementation to detach HEAD on Git versions prior
to 1.5.3. Nobody should be using such an old version anymore.
(Moreover, since 0730a5a3a, git-gui requires git v2.36 or later).
Keep only the code for modern Git.
Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
[j6t: message tweaked]
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>