[fix] support git versions <v2.22

LTS distros like Ubuntu 18.04 do not ship a up-to-date version of git.::

    $ sudo -H ./utils/lxc.sh cmd searx-ubu1804 git --version
    ...
    git version 2.17.1

The option `--show-current` was added in git v2.22, the alternative to this
option is::

    git rev-parse --abbrev-ref HEAD

Issue when using option `--show-current`::

    [searx-ubu1804] Clone searx sources
    [searx-ubu1804] -------------------
    [searx-ubu1804]
    [searx-ubu1804] error: unknown option `show-current'
    [searx-ubu1804] usage: git branch [<options>] [-r | -a] [--merged | --no-merged]
    ....

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>