git: Suppress stderr when running diff on Windows.

When using autocrlf, warnings
"LF will be replaced by CRLF in ..."
occur, causing the command window to pop up, which is not
desired.

Change-Id: I399080a98f9386dbbaff2c90c6d4ba4877d08057
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>