This gives auto-cancelling semantics on `Drop`, plus a nicer
`.commit()` method on the transaction.
Matches the currently private `_OstreeRepoAutoTransaction` in the C
library.
An intimidating spam of compiler errors at the start, but the
biggest was handling the new convention of `ostree_sys::` => `ffi::`.
This will require a semver bump of course.
Colin Walters
Felix Krull