build mode unimplemented

[nikomatsakis]

We need to make cargo incremental build work. My idea is roughly that it will do the following:

• abort if there are untracked .rs files or other things that look like they might affect the build
• let H be the current head
• git reset over to the cargo-incremental-build branch (creating it, if necessary)
• add all outstanding changes and commit into this branch with a msg like "checkpoint"
• git reset back to H
• run the build, using some incremental work-directory (which one?)

This should all be fairly straightforward using the git2 API.

[michaelwoerister]

This is implemented modulo bugs.