torture: Add script to smoke-test commits in a branch

This commit adds a kvm-check-branches.sh script that takes a list
of commits and commit ranges and runs a short rcutorture test on all
scenarios on each specified commit.  A summary is printed at the end, and
the script returns success if all rcutorture runs completed without error.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>