Test if it is possible to add files to the index of a bare repo.
Adding using a path should still require a non-bare repository.
Refactored the working tree dependent code for add.
Adding to a bare repository is now possible because @git_working_dir
is only used on the relevant methods.
Merge branch 'master' of https://github.com/moshevds/GitPython into moshevds-master

Fixed an issue with path_rewriter code branch, but there is more to do

Conflicts:
	git/index/base.py
	git/test/test_index.py
Fixed regression that would possibly have caused an abundance of chdir calls.
Fixed indentation error
Merge branch 'moshevds-master'