Fix win_git error message

library/win_git.ps1

@@ -93,7 +93,7 @@ function Get-GitRemoteHeadBranch {
     $result = Run-Command -command $command -working_directory $dest
     if ($result.rc -ne 0) {
         $module.FailJson("Could not determine the default HEAD branch of remote: $remote" ` +
-                         "$result.stdout $result.stderr")
+                         $result.stderr)
     }
     return $result.stdout.Trim().Replace("$remote/", '')
 }