Git: dereference topic result

sometimes tags are annotated, and have different hash than head - now
all will be good with them too.

Change-Id: Ibd66b78e6dbc7f181ba05f45339f2d97bb611602
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>