- Make hardlink handling more generic. The previous strategy worked for
  tar archives, but not for cpio. It now works for both.
- Add support for SEL labeling (through the OstreeRepoCommitModifier)
- Add support for xattr_callback (through the OstreeRepoCommitModifier)
- Add support for filter (through the OstreeRepoCommitModifier)
- Add a use_ostree_convention option

This was already supported by the commit modifier API, just needed to
expose it. This will also be used to test the libarchive API in a future
test.

- Test both tar and cpio archives
- Test more hardlink corner cases
- Test symlinks more rigorously
- Test stat override
- Test skip list

- Do a bit of refactoring
- Add test for use_ostree_convention
- Add test for xattr_callback
- Add test for SELinux labeling