Cope with multiple Pageant installs (one-line change)

If Pageant is installed multiple times (e.g. inside both TortoiseGit and TortoiseHg) then just pick the first one.

I actually fixed this on my installation as follows:
$pageant = Get-Command pageant -Erroraction SilentlyContinue | Select -ExpandProperty Name | head -1
but I think this pull request is the PowerShell way of doing the same thing.