Added support for separeted git dir.

[niyaton]

Git-init command supported --separate-git-dir option.
The repository initialized with this option manage the repository by using .git file instead of the .git directory.

I added support for these type repositories.

[jmlundberg]

I tried this and it work great when doing r=git.Repo().

It seems to not work for submodules: git.Repo().submodules[0].module_exists() still reports false if it uses a .git file. That is the case if git submodule update was first done using modern versions of command line git (at least 1.8.3.1 but likely earlier versions as well).

[Byron]

Thank you !
I had to make a minor for read_gitfile to actually work, and a test-case is yet to be written as well. Therefore I could only verify that it doesn't fail/break existing functionality.

@jmlundberg The submodule implementation has plenty of issues right now, the one you mention might be related to the 'new' way of storing submodules, which is not yet supported by the submodule implementation. These issues will be handled separately.