tests/test-username-newline.t

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).

  $ hg init
  $ touch a

  $ unset HGUSER
  $ echo "[ui]" >> .hg/hgrc
  $ echo "username= foo" >> .hg/hgrc
  $ echo "          bar1" >> .hg/hgrc

  $ hg ci -Am m
  adding a
  abort: username 'foo\nbar1' contains a newline
  
  [255]
  $ rm .hg/hgrc

  $ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
  abort: username 'foo\nbar2' contains a newline
  
  [255]
  $ hg ci -Am m -u "`(echo foo; echo bar3)`"
  transaction abort!
  rollback completed
  abort: username 'foo\nbar3' contains a newline!
  [255]