Support experiments generated from sampling a range of commits

[iand]

Create an experiment given start and end commits and n, the number of sample points. List the commits between start and end and divide into n+1 slices. Build kubo at the commit marking the end of slices 1-n. Run experiment with those builds.

For example, given commits a,b,c,d,e,f,g:

when n=1 we split into 2 (a,b,c,d) and (e,f,g) and run experiment with commit d
when n=2 we split into 3 (a,b,c), (d,e,f) and (g) would run with c and f
when n=3 we split into 4 (a,b), (c,d), (e,f) and (g) and would run with b, d and f