VCSDownloadStrategy: add last_commit method
VCSDownloadStrategy: add last_commit method

Implement:
  * VCSDownloadStrategy#last_commit
    Use last modified file timestamp
  * SubversionDownloadStrategy#last_commit
    Use `svn info --show-item revision`
  * GitDownloadStrategy#last_commit
    Use `git rev-parse HEAD`
  * MercurialDownloadStrategy#last_commit
    Use `hg parent --template {node}`
  * BazaarDownloadStrategy#last_commit
    Use `bazaar revno`
  * FossilDownloadStrategy#last_commit
  Use `fossil info tip`

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