ci: fix static checks merge base resolution

This change fetches the full history of the base branch and the change
under test, to avoid problems with unreachable commits.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I7ed38a65678552fff5b0f5efabf6bafc06ea86ee
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
14 files changed