Steps to reproduce

1. Add some .cs files with things you are working on that don't happen to be tracked yet.
2. Invoke Start-PSBuild -Clean to kick off a clean build process.
3. Your untracked files are deleted without warning or prompt, and they don't even go into your recycle bin.

Expected behavior

Start-PSBuild -Clean should not delete untracked source files without prompt by default (which git clean does). That is a really poor decision. I just lost a good chunk of work because of that behaviour, and now I'm losing time trying to get it back. All I wanted was a clean build (i.e. remove any temporary build files, and then rebuild). I did not want to remove files I happen to have in my file system that may be important to me.

Actual behavior

Any untracked files you have are removed without warning and without even going to the recycle bin.

Environment data

Irrelevant, but I happen to be invoking Start-PSBuild in Windows PowerShell 5.1.