Don't wait for pageant

It's supposed to be a background service style process, so this otherwise waits indefinitely.
Suspect this is true for other `$sshAgent`s too (see a few lines below) but have no test cases so haven't changed it.

GitUtils.ps1

@@ -303,7 +303,7 @@ function Start-SshAgent([switch]$Quiet) {
         $pageant = Get-Command pageant -TotalCount 1 -Erroraction SilentlyContinue
         $pageant = if ($pageant) {$pageant} else {Guess-Pageant}
         if (!$pageant) { Write-Warning "Could not find Pageant."; return }
-        & $pageant
+         Start-Process -NoNewWindow $pageant
     } else {
         $sshAgent = Get-Command ssh-agent -TotalCount 1 -ErrorAction SilentlyContinue
         $sshAgent = if ($sshAgent) {$sshAgent} else {Guess-Ssh('ssh-agent')}