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).
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