SourceTree 2.0 and Mercurial fail with SSH remotes

Summary

We had a Windows 7 developer using SourceTree 1.9.x with a Mercurial repository.    Commits were pushed and pulled to a repo on a linux box on our local network via ssh:

   ssh://linuxbox//path/to/repo

When they updated to SourceTree 2.0.20.1, the ssh remotes on the linux box stopped working.  We'd get a repeated "Windows Security" dialog for "Git Credential Manager for Windows" / "Enter your credentials for https://linuxbox/"  and username and password, but entering the proper values just keep cycling through the same dialog. 

Checking some details after cancelling showed Error reading SSH_ASKPASS output for prompt: Unsupported prompt

[login as: ]@linuxbox's password
abort: no suitable response from remote hg!

Reverting to SourceTree 1.9.9 and everything works fine again!  I did a very tiny test with embedded Git and 2.0.20.1, and it seems to get past the ssh connection properly, the problems seems limited to 2.0+Mercurial+ssh. 

Environment

The "General/SSH Client" preference is PuTTY/Plink, as it was under 1.9 (we have PuTTY 0.6.9). For the remote, we have a debian running mercurial 3.1.2 and ubuntu with mercurial 3.7.3, and SourceTree is using embeded Mercurial 3.7.3.