made shell script more POSIX compilant
forgot that perl is a prerequisite
docu states more explicitely that current working directory sadly is constrained to project's root
learned to cope with HEAD referencing a non-existing branch

This is the case after 'git init'. HEAD points to refs/heads/master, but refs/heads/master does not exist.
learned to display arrow to upstream branch
now displaying remote tracking branches with yellow fillcolor
redundant

exit is already called one line up.