Fixed issue with bz2 archive not being created.

Updated archive tests to check for non-zero file size.
 Merge branch 'fix/bzip2' into 'master'

Fix bz2 archive creation

I for some reason had a symlink named `bzip` when I created the initial `zip` and `bz2` archive support. The correct command should have been `bzip2`. This fixes the issue and updates the test to check for non-zero file size for the archives.

This was pointed out by @jacobvosmaer [here](https://github.com/gitlabhq/gitlab_git/commit/b571e1aca61a2915ec143b609d75657f1bc15de0#lib-gitlab_git-repository-rb-P17).