perf: factor selection of revisions involved in the merge out We will introduce more performance command around merge. As a first step we factor out pieces of `perfmergecalculate` that can be reused.

perf: allow to specify the base of the merge in perfmergecalculate We can now test the rebase case.

perf: add a --from flag to perfmergecalculate Before this change, `perfmergecalculate` was always benchmarking the merge of the working copy with another revision. We can now benchmark the `mergecalculate` call for any arbitrary pair of revision.