Allow to manually convert a project without having to re-create it

I understand you might have good reasons not to automatically convert repositories from hg to git and vice-versa, however having to delete and re-create a whole repository is not really optimal: that could be improved.

You could allow us to change repository type, deleting all the sources, but leaving intact the collaborators, issues and wiki; we would locally manage repository conversion and upload it again, without having to worry about all the rest.