static-checks: use origin/master instead of master as base branch

In some CI testing scenarios, there are no local branches, just remotes, so
as a safer approach, use 'origin/master' instead of 'master'. Base
branch is used to find common parent ancestor between
base (origin/master) and user patchset's (FETCH_HEAD) branches.

Change-Id: I36940ac043f45bda6b94e8787b29f319a5f4acba
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
4 files changed