Improvements to built-in vcs integration

[silasary]

MSE has VCS support. But it sucks.

My intent with this PR is to overhaul it a bit:

• Add Git support alongside svn.
• Automatically commit, rather than just staging changes.
• Pull when loading a set (I think it's possible, but we'll see)
• Automatically detect if a set is under version control, rather than having a property in set.
• Maybe add a preference for whether a user wants commit messages or an auto-generated message?
• Maybe make a MSE specific commit message window, rather than whatever the user set EDITOR to (important if it's vim or some other console-based editor)