Improve support of incremental copy

[candera]

Background

Currently, when making a full copy, hobocopy simply records the time of the copy in statefile. When doing an incremental copy, the copy simply looks at each file, and copies it if it has been modified after the date in the statefile. That's not really right, though, as things like moving a directory or a file don't necessarily change the date, but should be reflected in the incremental copy somehow. The issue with this is that when a user restores the full backup and then the incremental one, they could wind up with two copies of some files. Worse, one of them will be old.

Expected Behavior

• Users should be able to reproduce the state of the backed-up directory using the full backup and an incremental one.

Open Questions

• What should we do about deleted files? Perhaps a list of deleted files should be produced. What do other backup tools do?