git-gui: use HEAD as current branch when detached

git-gui: use HEAD as current branch when detached

commit f87a36b697 ("git-gui: use git-branch --show-current", 2024-02-12)
changed git-gui to use git-branch to access refs, rather than directly
reading files as doing the latter is not compatible with the reftable
backend. git branch --show-current reports an empty branch name when the
head is detached, and in this case load_current_branch needs to report
HEAD using special case logic as it did prior to the above commit. Make
it do so.

This addresses an issue with git-gui browser failing with a detached
head.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>