Rewrite commit coalescing logic to preserve monotonicity

Rewrite commit coalescing logic to preserve monotonicity

The previous logic would allow a situation such as A A' B B' C C'
where A' B' and C' could be merged to result in A A* B C violating
history flow for files in B and C.

The new strategy coalesces less commits but the result is (or should be
modulo new bugs) correct.