Search parent directories for .git dir

I ran into the issue described in ktoso/maven-git-commit-id-plugin#113 today. While I like the rev-parse idea and have seen that implemented in other places, I just opted to search parent directories until I find a .git directory within one.

To keep the tests passing, I had to modify them to use a different sandbox directory--one not parented by the project, otherwise the recursive parent calls would pick up the maven-git-command-id-plugin's git directory mistakenly.

This is working great for me now, but I welcome feedback.