perf: add perfchangegroupchangelog command This command can be used for testing the performance of producing the changelog portion of a changegroup. We could use additional perf* commands for testing other parts of changegroup. Those can be written another time, when they are needed. (And those may want to refactor the changegroup generation API so code can be reused.) Speaking of code reuse, yes, this command does reinvent a small wheel. I didn't want to scope bloat to change the changegroup API because that will invite bikeshedding.