Don't rmtree tmp #56
Description
I was running prm inside a docker container, with my working directory set to /....
And spent a long time trying to figure out why my files in /tmp kept disappearing:
https://github.com/dnbert/prm/blob/master/lib/prm/repo.rb#L139
It would be nice if all of the temp directory stuff created its own tempdir, and didn't hard code the relative path tmp/