R1:6615d76c3489

In Subversion, show "svn checkout <uri> <directory>" in Diffusion

Summary:
Ref T10923. The old behavior was to show a full command in SVN, Mercurial, and Git, like this:

- `git clone <uri>`
- `hg clone <uri>`
- `svn checkout <uri> <directory>`

In Git and Mercurial, the `<uri>` ends in something like `/nice-repository-name.git` so the default directory it creates is called `nice-repository-name/`.

In Subversion, we don't (and can't easily) do that for various reasons so we provide an explicit…