Git: Use Git process environment for Gerrit plugin processes

Change-Id: Iab0124d3aeed189c5ffd900c02ce6b87293b9c79
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Git: Set HOME variable by default on Windows

Change-Id: Ib780f794acfc58753b139174ea3ea90bee542a38
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>