[String]$buildID = "$env:BUILD_BUILDID"
[String]$project = "$env:SYSTEM_TEAMPROJECT"
[String]$projecturi = "$env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI"

$username="alternativeusername"
$password="alternativepassword"

$basicAuth= ("{0}:{1}"-f $username,$password)
$basicAuth=[System.Text.Encoding]::UTF8.GetBytes($basicAuth)
$basicAuth=[System.Convert]::ToBase64String($basicAuth)
$headers= @{Authorization=("Basic {0}"-f $basicAuth)}

$url= $projecturi + $project + "/_apis/build/builds/" + $buildID + "/changes?api-version=2.0"
Write-Host $url

$responseBuild = Invoke-RestMethod -Uri $url -headers $headers -Method Get | select value

foreach ($change in $responseBuild.value)
{
    $commitmessage = $change.message
    Write-Host $commitmessage
}