Log versions of code whenever the autograder is run

[sravanareddy]

We currently log every output produced by the autograder when the student runs it in a .logs directory for further analysis. It may be useful to log the code as well, both for pedagogical analysis and for detection of collaboration, help, and so on -- a sort of version-control-lite.

[bpw]

This sounds very useful. Especially for the similarity detection -- could run moss on intermediate versions.

Presumably if students are just uploading their whole psXX directory, we would transparently get access to those versions without any sort of extra network effort.