Default clone_repo to master if branch not found

Behavior change from set -e causes attempt to checkout an invalid
branch to fail. Explicitly fall back to master if the requested
branch name checkout fails.

Change-Id: I5d1f6a997375743bb3498e7fae1fbad19c307d33

tools/clone_repo.sh

@@ -169,6 +169,6 @@ else
     # directory already existed we might have checked out something else
     # before so just do it again).
     (cd $local_dir &&
-            git checkout $BRANCH &&
+            (git checkout $BRANCH || git checkout master) &&
             git pull --ff-only)
 fi