Launch gitk only for current file,
or for the folder that current file belongs to

Task-number: QTCREATORBUG-8327

Change-Id: I2ffbd44e3b3eeaffad32e84af9ca2085b16fdaa2
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>