A posh-git script that identifies local branches that have been merged to `master`.

Remove-GitMergedBranches.ps1

function Remove-GitMergedBranches {
    Write-Output "Fetching latest"
    git fetch origin --prune
    
    $currentBranch = & git rev-parse --abbrev-ref HEAD
    if ($currentBranch -ne "master") {
        Write-Output "Changing branch to master"
        git checkout master
    }

    # Merge with origin with **fast-forward only** so that `git` will stop if there is an error
    Write-Output "Fast-Forward merge with origin/master"
    git merge origin/master --ff-only
    if ($LASTEXITCODE -ne 0) {
        throw "git exit code: $LASTEXITCODE"
    }

    git branch | ? { $_ -notmatch '(^\*)' } | ForEach-Object {
        $branch = $_.trim()
        Write-Output "$branch"
        try {
            git merge --no-ff --no-commit $branch | Out-Null
            git diff --staged --exit-code --quiet
            if ($LASTEXITCODE -eq 0) {
                Write-Output "Would delete '$branch'"
                # git branch -D $branch
            }
        } finally {
            git merge --abort | Out-Null
        }
    }
}