tools/check-commits.sh: use PR head instead of base
tools/check-commits.sh: use PR head instead of base

Base points to already commetted changes,
we really want to cheack head of the PR.

google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller