update-test: allow setting start commit
update-test: allow setting start commit

To use origin/master as start commit
  brew update-test

To use <sha1> as start commit
  brew update-test --commit=<sha1>

To use commit at <date> as start commit
  brew update-test --before=<date>

Also print start/end commit for debug purpose

MikeMcQuaid/brew,mgrimes/brew,vitorgalvao/brew,pseudocody/brew,sjackman/homebrew,DomT4/brew,konqui/brew,nandub/brew,mgrimes/brew,Linuxbrew/brew,zmwangx/brew,reitermarkus/brew,DomT4/brew,toonetown/homebrew,reelsense/brew,ilovezfs/brew,Homebrew/brew,maxim-belkin/brew,nandub/brew,vitorgalvao/brew,palxex/brew,maxim-belkin/brew,palxex/brew,pseudocody/brew,ilovezfs/brew,tonyg/homebrew,claui/brew,tonyg/homebrew,amar-laksh/brew_sudo,sjackman/homebrew,nandub/brew,JCount/brew,MikeMcQuaid/brew,ilovezfs/brew,DomT4/brew,amar-laksh/brew_sudo,gordonmcshane/homebrew,jmsundar/brew,MikeMcQuaid/brew,EricFromCanada/brew,toonetown/homebrew,mahori/brew,reelsense/brew,Linuxbrew/brew,gregory-nisbet/brew,muellermartin/dist,sjackman/homebrew,rwhogg/brew,konqui/brew,muellermartin/dist,alyssais/brew,reelsense/brew,claui/brew,EricFromCanada/brew,JCount/brew,mahori/brew,claui/brew,hanxue/linuxbrew,Linuxbrew/brew,tonyg/homebrew,mgrimes/brew,muellermartin/dist,toonetown/homebrew,nandub/brew,gregory-nisbet/brew,JCount/brew,sjackman/homebrew,mistydemeo/homebrew,EricFromCanada/brew,Homebrew/brew,amar-laksh/brew_sudo,staticfloat/brew,Homebrew/brew,Homebrew/brew,mistydemeo/homebrew,mistydemeo/homebrew,jmsundar/brew,mahori/brew,aw1621107/brew,pseudocody/brew,AnastasiaSulyagina/brew,hanxue/linuxbrew,MikeMcQuaid/brew,gregory-nisbet/brew,jmsundar/brew,rwhogg/brew,AnastasiaSulyagina/brew,hanxue/linuxbrew,konqui/brew,aw1621107/brew,DomT4/brew,alyssais/brew,reitermarkus/brew,staticfloat/brew,bfontaine/brew,zmwangx/brew,rwhogg/brew,reitermarkus/brew,JCount/brew,Linuxbrew/brew,aw1621107/brew,bfontaine/brew,AnastasiaSulyagina/brew,maxim-belkin/brew,palxex/brew,konqui/brew,staticfloat/brew,alyssais/brew,claui/brew,EricFromCanada/brew,bfontaine/brew,reitermarkus/brew,gordonmcshane/homebrew,mahori/brew,zmwangx/brew,gordonmcshane/homebrew,vitorgalvao/brew,vitorgalvao/brew