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.

perf: add --reverse to perfrevlog It can be useful to know how fast we can read revisions from a revlog in reverse. This operation tends to occur in `hg log` commands, for example.

log: copy the way of ancestor traversal to --follow matcher (issue5376) We can't use fctx.linkrev() because follow() revset tries hard to simulate the traversal of changelog DAG, not filelog DAG. This patch fixes _makefollowlogfilematcher() to walk file ancestors in the same way as revset._follow(). I'll factor out a common function in future patches.

log: unroll loop that populates file paths for --patch --follow matcher We can't handle the first fctx in the same manner as its ancestors. Also, I think the original code was too tricky.

wireproto: rename argument to groupchunks() groupchunks() is a generic "turn a file object into a generator" function. It isn't limited to changegroups. Rename the argument and update the docstring to reflect this.

tests: actually test non-generaldelta variant for stream clones 608cabec1b15 accidentally made both tests test generaldelta repos. Restore the test for a non-gd repo.