Support absoulte path in git submodule gitdirs

When using git submodules, some versions of git (e.g., 1.7.9) use
abosulate path in the gitdir reference of the submodules. This patch
adds support to handle absoluate path correctly.
Use parent directory not parent project to locate gitdir

When searching for the git directory of a project the search follows
the project's basedir parent directory rather than following the
project hierarchy (which is not correct)
Merge pull request #135 from maiergre/master

Improve gitdir handling with submodules