Fix bug in Invoke-Git

[EklipZgit]

Fix to prevent the following from occurring due to $stderr sometimes being an array due to lines 124-130:

2019-03-13T16:36:28.5845108Z ##[error]Write-Error : Cannot convert 'System.Object[]' to the type 'System.String' required by parameter 'Message'. Specified method is not supported. At C:\Program Files\WindowsPowerShell\Modules\BuildHelpers\2.0.7\Public\Invoke-Git.ps1:149 char:29

• Write-Error $stderr.trim()

• 
  + CategoryInfo          : InvalidArgument: (:) [Write-Error], ParameterBindingException
  + FullyQualifiedErrorId : CannotConvertArgument,Microsoft.PowerShell.Commands.WriteErrorCommand

[RamblingCookieMonster]

Good catch, thanks!!