Retrieve changes from commit in Azure DevOps pipeline

I want to create the following solution:

• A new branch is created and a subfolder + configuration file is added
• The changes are committed to the branch
• An ADO pipeline is triggered by the commit
• The pipeline retrieves the name of the added subfolder and content

On my machine this configuration works:

{
- task: PowerShell@2
    displayName: 'Retrieve subFolderName'
    inputs:
        targetType: 'inline'
        script: |
            $changedFiles = git diff main --name-only --relative -- "**/myDefinition.json"
            $folders = ""
            Write-Verbose "ChangedFiles:" -Verbose
            $changedFiles | Out-String | Write-Output

            foreach ($file in $changedFiles) {
                Write-Verbose "File: $file" -Verbose
                if ($file -like "*/myDefinition.json") {
                $folders = Split-Path -Parent $file
                }
            }

            if ($folders) {
                Write-Output "Path to myDefinition.json: $folders"
                $subFolderName = $($folders.split('\'))[-1]
                Write-Output "SubscriptionName: $subFolderName"
            }
            else {
                Write-Error "No myDefinition.json found in the commit."
            }

However, when I run it in the pipeline, I get this error message:
fatal: bad revision 'main'

I tried to add git fetch, but then I get: fatal: could not read Password for <>

Is git diff the way to go here, or should I use a different command?

After a lot of iterations, I finally got this code to work:

trigger:
  branches:
    include:
    - alz/*

pool:
  vmImage: 'ubuntu-latest'

steps:
  - checkout: self
    persistCredentials: true

  - task: PowerShell@2
    displayName: 'Retrieve MyDefinition.json'
    inputs:
        targetType: 'inline'
        script: |
            echo ======Git-Version=====
            git --version
            echo ======Git-Branch======
            git branch
            echo ======BuildVersion======
            echo $(Build.SourceVersion)
            echo ======Git-Fetch===========
            git fetch origin "+refs/heads/*:refs/remotes/origin/*"
            echo ======Checkout-Branch===========
            git checkout -B main origin/main
            echo ======Git-Branch======
            git branch
            echo ======Git-Status======
            git status
            echo ======Git-Diff======
            git diff $(Build.SourceVersion) --name-only --relative
            echo ======Script======
            $changedFiles = git diff $(Build.SourceVersion) --name-only --relative -- "**/MyDefinition.json"
            $folders = ""
            Write-Verbose "ChangedFiles:" -Verbose
            $changedFiles | Out-String | Write-Output

            if($changedFiles -ne $null) {
              foreach ($file in $changedFiles) {
                  Write-Verbose "File: $file" -Verbose
                  if ($file -like "*/MyDefinition.json") {
                  $folders = Split-Path -Parent $file
                  }
              }
            }
            else {
              Write-Warning "No MyDefinition.json found in the commit!"
            }

            if ($folders) {
                Write-Output "Path to MyDefinition.json: $folders"
                $subFolderName = $($folders.split('/'))[-1]
                Write-Output "SubscriptionName: $subFolderName"
            }
            else {
                Write-Warning "No MyDefinition.json found in the commit."
            }