static-checks: use `remotes/origin/master` instead of `master`

This is a safer approach in cases where master local branch does not
exist, i.e. shadow cloning.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I2fdae51673169d4c32d3f308c2eb3fd885c42e76
diff --git a/script/static-checks/check-copyright.py b/script/static-checks/check-copyright.py
index 39863c7..63db460 100755
--- a/script/static-checks/check-copyright.py
+++ b/script/static-checks/check-copyright.py
@@ -180,7 +180,7 @@
 only files that are modified by the latest patch(es).""",
                         action="store_true")
 
-    (rc, stdout, stderr) = utils.shell_command(['git', 'merge-base', 'HEAD', 'master'])
+    (rc, stdout, stderr) = utils.shell_command(['git', 'merge-base', 'HEAD', 'refs/remotes/origin/master'])
     if rc:
         print("Git merge-base command failed. Cannot determine base commit.")
         sys.exit(rc)