Add postBuffer to gitconfig

We need to set postBuffer to 512MB to prevent
"the remote end hung up unexpectedly" happening during git clone.

Change-Id: I5e272304e4f1d6db42663732c2318e685263128f
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>