* the `get_path_history` is now sub-repository aware.
A few refactorings has been done to clarify the code:
* backported the `Repository.has_node(path,rev)` method from TracDiff
* introduced a `_scoped_path(scope, fullpath)` function which
truncates the scope from a full repository path, as returned by the svn API
* simplified the `_normalize_path` function
* added a bunch of unit-tests to cover the `versioncontrol` API
in the sub-repository case
* fix `log:@123:234` links (i.e. if no path is given, take the root)
This fixes #1930.
