Optimization: provide static detect presense method to avoid Client init

Also removes redundant call to os.path.exist(parentdir)

Explanation: currently vcstools does not offer to detect the presence of a vcs without initializing a vcsclient, which always implies a shell command and thus wastes time. With this change, clients can instead call a static method to decide whether they need a client in the first place.

wstool has to be adapted later to use this changed API for a slight speed benefit, see wstool/src/wstool/conf_element.py:AVCSConfigElement