The subprocess.Popen() does not change the child process's working
directory if cwd=None is given.  Let's exploit this fact to refactor
the source directory handling.

We no longer have to pass "-C <reference_src_dir>" to the sub-process
because self.current_src_dir tracks the source tree against which we
want to run defconfig/autoconf.

The flag self.use_git_ref is not necessary either because we can know
the current state by checking whether the self.current_src_dir is a
valid string or None.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>