List all branches of forks from a GitHub repository that don't exist on the root repository

github_forks_branches.ps1

param(
    [Parameter(Mandatory=$true)]
    [string]$rootRepo
)
$gh = 'C:\Program Files (x86)\GitHub CLI\gh.exe'
$rootBranches = (. $gh api /repos/$rootRepo/branches | ConvertFrom-Json).name
$repos = . $gh api /repos/$rootRepo/forks | ConvertFrom-Json
$repos | ForEach-Object {
    $repo = $_.full_name
    $branches = (. $gh api /repos/$repo/branches | ConvertFrom-Json).name
    $differences = Compare-Object -ReferenceObject $rootBranches -DifferenceObject $branches |
        Where-Object { $_.SideIndicator -eq '=>' } | 
        Select-Object InputObject,@{n='URL';e={ "https://github.com/$repo/tree/$($_.InputObject)" }}
    if ($differences.Length -gt 0) {
        Write-Host -ForegroundColor Green $repo
        $differences | Format-Table -HideTableHeaders
    }
}