download_strategy: use short hash for git last_commit
download_strategy: use short hash for git last_commit

Closes #460.

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

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