extract: fix corner case
extract: fix corner case

If the formula to be retrieved was just version-bumped in the most
recent commit (HEAD), we would've ended up grabbing the second-most
recent version.

Instead, if the file already exists in tree at the current commit,
just construct the formula from that to get the version (for naming
purposes) and copy the file outright to its new location.

claui/brew,nandub/brew,JCount/brew,maxim-belkin/brew,mahori/brew,Homebrew/brew,reitermarkus/brew,reitermarkus/brew,Homebrew/brew,konqui/brew,mahori/brew,nandub/brew,nandub/brew,EricFromCanada/brew,mahori/brew,MikeMcQuaid/brew,JCount/brew,sjackman/homebrew,claui/brew,claui/brew,maxim-belkin/brew,vitorgalvao/brew,Linuxbrew/brew,vitorgalvao/brew,Linuxbrew/brew,EricFromCanada/brew,konqui/brew,konqui/brew,sjackman/homebrew,DomT4/brew,JCount/brew,mahori/brew,maxim-belkin/brew,vitorgalvao/brew,EricFromCanada/brew,claui/brew,vitorgalvao/brew,Homebrew/brew,MikeMcQuaid/brew,sjackman/homebrew,reitermarkus/brew,DomT4/brew,MikeMcQuaid/brew,sjackman/homebrew,Homebrew/brew,DomT4/brew,DomT4/brew,Linuxbrew/brew,nandub/brew,konqui/brew,Linuxbrew/brew,EricFromCanada/brew,JCount/brew,MikeMcQuaid/brew,reitermarkus/brew