Fix directory/path manipulation methods

Fix directory/path manipulation methods

The `dirname` and `dirbase` methods have been replaced with the Android
implementation, which is actually compilant to some kind of standard.

A new method `topdir` has been added, which returns the topmost
directory in a path.

These changes fix issue #49:

	`gitfo_prettify_dir_path` converts "./.git/" to ".git/", so
	the code at src/repository.c:190 goes out of bounds when
	trying to find the topmost directory.

	The new `git__topdir` method handles this gracefully, and the
	fixed `git__dirname` now returns the proper value for the
	repository's working dir.

	E.g.

		/repo/.git/ ==> working dir '/repo/'
		.git/		==> working dir '.'

Signed-off-by: Vicent Marti <tanoku@gmail.com>