Rename the branch "root" to "master" if the latter isn't used

When the repository uses the canonical namespaces structure, commits to the repository root can be considered as mistakes, and should be moved to a special "root" branch. If instead the structure is not used and everything is committed in the repository root, there is no need to have a separate "root" branch. Map back the "root" branch to "master" in that case.

This is a convenience for very simple repositories that should have been read with the --nobranch option. The latter will do the right thing in case of flat repositories even if they happen to have a trunk subfolder.

This also helps keeping the result close to the original svn reader which did that.