Update-FromGitHubUpstream.ps1

Set-StrictMode -Version 2
$ErrorActionPreference = 'Stop'

function Write-ColorOutput([Parameter(Mandatory=$true)] $inputObject, [System.ConsoleColor] $foregroundColor) {
    $rawUI = $Host.UI.RawUI    
    $saved = $rawUI.ForegroundColor
    try {
        $rawUI.ForegroundColor = $foregroundColor
        Write-Output $inputObject
    }
    finally {
        $rawUI.ForegroundColor = $saved
    }
}

Write-ColorOutput 'fetching origin' -ForegroundColor White
git fetch origin
Write-ColorOutput 'fetching upstream' -ForegroundColor White
git fetch upstream

$branches = git branch --all | % { $_ -replace '^[\s\*]*|\s+.+$','' }
$local = $branches | ? { $_ -notmatch '^remotes/' }
$upstream = $branches | ? { $_ -match '^remotes/upstream/(\S+)' } | % { $matches[1] }

$branches |
  ? { $_ -match '^remotes/origin/(.+)' } |
  % { $matches[1] } |
  ? { $_ -ne 'HEAD' } |
  sort -unique |
  ? { ($local -contains $_) -and ($upstream -contains $_) } |
  % {
      Write-ColorOutput "$($_):" -ForegroundColor White
      Write-ColorOutput "  checking out" -ForegroundColor White
      git checkout $_

      Write-ColorOutput "  fast-forwarding" -ForegroundColor White
      git merge upstream/$_ --ff-only
  }