This matches the behavior before 3.4.0.
Silent errors from ls-files to avoid warnings about e.g. directories that
aren't readable and also list files that would have been encrypted had they not
been tracked in git (#521).
Fix the patterns written to info/exclude so that they match the same files as
are encrypted (e.g. *.key should only match .key files in the topdir, not in
subdirs).
The new test system is written with py.test. These tests are more
comprehensive, run faster by an order of magnitude, and are far more
maintainable. The tests themselves conform to PEP8.
