setup_git_directory: make sure GIT_DIR is a valid repository.

setup_git_directory: make sure GIT_DIR is a valid repository.

setup_git_directory() always trusted what the user told where
GIT_DIR was, and assumed that is a valid .git/ directory.  This
commit changes it to at least do the same level validation as
is_toplevel_directory() does -- has refs/, has objects/ unless
GIT_OBJECT_DIRECTORY is set, and has valid HEAD symlink or
symref.

Signed-off-by: Junio C Hamano <junkio@cox.net>