Example:
{{{
- trunk
- projA
- branches
- 1.0
- projA
}}}
Here, `branches/1.0` was copied from `trunk` and we're interested in the `svn:mergeinfo` property on `branches/1.0/projA`.
The previous way of computing `branch_starts` was broken in this case (`closest_copy('branches/1.0/projA')` returns `'branches/1.0'`) and fixing `SubversionNode.get_copy_origin` proved to be too cumbersome.
Instead, implement a `SubversionNode.get_copy_ancestry` which yields directly the whole copy ancestry. From that point, we get the `branch_starts` information, taking the latest range from a given source path.
