Verify permissions in check_repo_or_die()

Verify permissions in check_repo_or_die()

Currently, if one doesn't have read or access permission up to
repo('objects/pack'), bup exits with the following error:

error: repo() is not a bup/git repository

(with repo() replaced with the actual path).

This is misleading, since there is possibly really a repository there
but the user can't access it.

Make git.check_repo_or_die() verify that the current user has the
permission to access repo('objects/pack'), and if not, output a
meaningful error message.

As a bonus, we get an error if the bup_dir path is not a directory.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>