Currently 'clean after checkout' does 3 things:
1) Delete un-ignored untracked files and directories (similar to git clean -df)
2) Delete ignored untracked files and directories (similar to git clean -xdf)
3) Reset tracked files (similar to git reset)

I would like to only do #3. It would also be useful to do #1 and #3 without doing #2.

This is because we are using Unity which creates a lot of library files which we are happy to persist between builds. These files are required and take a long time to generate but do not affect the build outcome so are safe to leave. The file path is added to .gitignore.

It may make sense to add a 'clean' behaviour and a 'reset' behaviour with somewhere we can pass our own parameters.