update-bash: fix revision variable setting
update-bash: fix revision variable setting

* only set HOMEBREW_UPDATE_BEFORE inside pull instead of fetch.
* fix HOMEBREW_UPDATE_BEFORE/AFTER variable settings. They should be set
  to INITIAL_REVISION and CURRENT_REVISION correspondingly.
* avoid unnecessary duplicated shellout.
* remove unused variable.

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