Improve the clean and rebuild section.
Improve the clean and rebuild section.

See #194. [skip ci]
sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian,bgamari/shaking-up-ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc