jit-diff: handle output better

jit-diff outputs quite a lot of text (such as the command lines for every call to jit-dasm). If any warnings or errors are output before or during a run, they can be overwhelmed by the "normal" output. This can be solved in one of several ways:

1. Display so much less output by default that any warning/error output is easily seen. (All output could be displayed by passing a `--verbose` switch).
2. Save all warning/error output, and re-display it at the very end (like the way msbuild does)