Push some more behaviour to the vc backend

[Lele Gaifax]

In order to get my new TracDarcs plugin working, I had to apply this patch that

1. Fixes #2731
2. Adds an "hash" field to the revision table
3. Moves the actual insertion of the cache data from the CachedRepository class to the Changeset one

Point 3 allows the backend to augment the cached data as needed, simply by overriding a Changeset method (see for example).

Of course, I could have reimplemented the whole sync() machinery in a subclass of CachedRepository, but DRY approach suggests that's not the right way.

[mgood]

It would help if you explained the purpose of the "hash" column since it's not actually used in the patch anywhere.

What changes are necessary in the cache? The classes in trac.versioncontrol.api are meant to be decoupled from the cache, so if it's necessary to extend the cache, this should be done in the cache module, not api.

[cboos]

See VcRefactoring