A more reliable way to check if a configured git repository exists and actually accessible

Review Request #871 — Created May 20, 2009 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
This change allows to check if git actually installed even if os.environ doesn't contain PATH which includes a directory with git binary.
Such situations happen e.g. in [some] Apache/mod_fastcgi setup (I guess an actual PATH is unavailable from within fcgi-scripts due to security reasons or whatever)