Git's config is UTF-8 encoded on Windows

'git config' command returns UTF-8 strings if the encoding of the file was not hijacked by the user.

Task-number: QTCREATORBUG-8696

Change-Id: I6181a26e56e2883ad5d46194f974e799bfd2aafb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>