debugbuilddag: use memctx for speed This drops the options to run arbitrary shell commands within commits and the option to create a file that gets appended to in every revision. It now supports to not write file data at all, which is very fast for generating a pure 00changelog.i (useful for discovery tests, for instance). Timings for 1000 linear nodes: * Old `hg debugbuilddag -o '+1000'` took 4.5 secs. * New `hg debugbuilddag -o '+1000'` takes 2 secs. * New `hg debugbuilddag '+1000'` takes 0.8 secs. (The last one creates only 00changelog.i).