pull: fix head_revision definition.
pull: fix head_revision definition.

Fixes #7405.

EricFromCanada/brew,Homebrew/brew,Homebrew/brew,sjackman/homebrew,MikeMcQuaid/brew,nandub/brew,Homebrew/brew,sjackman/homebrew,MikeMcQuaid/brew,claui/brew,MikeMcQuaid/brew,claui/brew,mahori/brew,reitermarkus/brew,nandub/brew,vitorgalvao/brew,claui/brew,EricFromCanada/brew,reitermarkus/brew,sjackman/homebrew,vitorgalvao/brew,JCount/brew,konqui/brew,MikeMcQuaid/brew,nandub/brew,Homebrew/brew,vitorgalvao/brew,JCount/brew,konqui/brew,claui/brew,mahori/brew,reitermarkus/brew,EricFromCanada/brew,JCount/brew,mahori/brew,sjackman/homebrew,konqui/brew,vitorgalvao/brew,reitermarkus/brew,mahori/brew,konqui/brew,EricFromCanada/brew,nandub/brew,JCount/brew