Improvements to `brew versions`

The output happens as it is determined, rather than all at once, which is preferable for CLI commands. This meant I had to hard-code the left-justification.

The pre-amble warning is removed. Instead the command outputs the git checkout command for each she that you can type to get that old formula.

I decided that reseting the one file will work 99% of the time and that it is too risky to reset the whole repo, this will roll-back bug fixes. Instead we should add functionality to compat/ in order to support old formula as required. Sometimes deps may have to be rolled-back too, but the user will have to figure this out on an ad-hoc basis. We are assuming a failure sophisticated user anyway, one who would like to get old versions of stuff.

I moved most of the functions into Formula since IMO this makes more conceptual sense.

I made the she get abbreviated by git (using --abbrev-commit) itself so it as short as can be.

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