`brew-unpack` now runs `Formula.brew` and copies the stage directory to the
unpack location. This provides two improvements:
- Patches may be applied. The `patch` metho of the Formula class is
overloaded such that patches are only applied if the `--patch` option is
passed to `brew unpack`.
- Formula can respond better to flags passed by `ARGV`.
Some trickery is pulled to ensure `DATA` is correctly set for each unpacked
formula.
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