update: abbreviate hashes to at least 7 characters
update: abbreviate hashes to at least 7 characters

Abbreviations will be longer if needed to preserve uniqueness.

This makes it more consistent with `git rev-parse --short`,
https://github.com/Homebrew/homebrew/commits, etc.

For example, instead of:

    Updated Homebrew from 40d1e9c2 to 90b9bdf4.

We see:

    Updated Homebrew from 40d1e9c to 90b9bdf.

See 0c48248b6555353c8d60b61973639c8423ce586c for the original
introduction of eight-character abbreviations.

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