fix update-test
fix update-test

Previously, `brew update-test` is run against master branch of local
repo. However, we test PR using a detached branch in `brew test-bot`.
The result is `brew update-test` will always be up-to-date in `test-bot`.

To fix it, we create two local copies of git repo, and set master branch
to start and end sha1 correspondingly. After that, `brew update` will be
run to simulate the change between start and end sha1.

Closes Homebrew/homebrew#43902.

Signed-off-by: Xu Cheng <9a05244150b861e40c78843b801eed71bca54eac@me.com>

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