This adds a tools/ script that checks any cherry-picked hashes
on the current commit (or a provided commit) to make sure that
all the hashes exist on at least master or stable/.* branches.
This should help avoid accidentally merging stable backports
where one of the hashes along the line has changed due to conflicts.
Change-Id: I4afaa0808b75cc31a8dd14663912c162281a1a42
aebc829c4e