1398 - Improve compatibility with older git(1) versions

Commands used for pull, update and get revision are modified to improve backwards compatibility. 
At least here `--git-dir` is not enough, we would have to set `--work-tree` as well (everything else operates just in `.git`, but `checkout` updates the working directory resp. the path where the dependency resources have been cloned into). Wladimir suggested avoiding `--git-dir` and `--work-tree` in general and use the `cwd` keyword-argument, but this seems to be not available for the `subprocess.check_output()` function. Any ideas?