def remote_branch_revs
    out = execute "git show-ref"
    out.split("\n").collect { |l|
      if l =~ /refs\/remotes\//
        l.split[0]
      else
        nil
      end
    }.compact
  end
def remote_branch_revs
    out = execute "git show-ref"
    out.split("\n").collect { |l|
      if l =~ /refs\/remotes\//
        l.split[0]
      else
        nil
      end
    }.compact
  end
returns the SHA-1 representations of the heads of all remote branches