pull: warning if we are not in master
pull: warning if we are not in master

Closes Homebrew/homebrew#39708.

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

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