StartBuildOnlyCommitNoBranch.ps1

$gh_user = "<github_user>"
$gh_repo = "<github_repo>"
$sha = "<your_sha>"
$av_account = $gh_user # not always true
$av_project_slug =  $gh_repo # not always true
$av_api_token = "<your_appveyor_api_token>"

# not officially supported way to get branch
# add authorization header for private repo
$resp = Invoke-WebRequest -Uri "https://github.com/$gh_user/$gh_repo/commit/$sha"
$html = New-Object -ComObject "htmlfile"
$html.IHTMLDocument2_write($resp.Content)
$branch = ($html.all.tags("li") | Where-Object {$_.className -eq "branch"}).innerText
Write-host "branch: $branch"


$headers = @{
  "Authorization" = "Bearer $av_api_token"
  "Content-type" = "application/json"
}

$body = @{
    accountName=$av_account
    projectSlug=$av_project_slug
    branch=$branch
    commitId=$sha
}
$body = $body | ConvertTo-Json

Invoke-RestMethod -Uri 'https://ci.appveyor.com/api/builds' -Headers $headers  -Body $body -Method POST