Improve gitdir handling with submodules

Thanks for the plugin! 

This pull req fixes two issues I encountered:
* When using git submodules some older git version (e.g., 1.7.9, used by Ubuntu 12.04 LTS) use absolute path in the .git gitdir reference. I've added support for absolute path.
* When searching for the gitdir the plugin should search the parent *directory* not the parent project. Esp. with submodules searching the parent project is not correct since the parent might be in a different git repo. 
gitdir is always `!= null` here