CONTRIBUTING

annotate: do not construct attr.s object per line while computing history

Unfortunately, good abstraction has a cost. It's way slower to construct
an annotateline() object than creating a plain tuple or a list. This patch
changes the internal data structure from row-based to columnar, so the
decorate() function can be instant (i.e. no Python in hot loop.)

For code readability, the outermost tuple is switched to an attr.s object
instead.

  (original, row-based attr.s)
  $ hg annot mercurial/commands.py --time > /dev/null
  time: real 11.470 secs (user 11.400+0.000 sys 0.070+0.000)
  $ hg annot mercurial/commands.py --time --line-number > /dev/null
  time: real 39.590 secs (user 39.500+0.000 sys 0.080+0.000)

  (this patch, columnar)
  $ hg annot mercurial/commands.py --time > /dev/null
  time: real 11.780 secs (user 11.710+0.000 sys 0.070+0.000)
  $ hg annot mercurial/commands.py --time --line-number > /dev/null
  time: real 12.240 secs (user 12.170+0.000 sys 0.090+0.000)

  (cf. 4.3.3, row-based tuple)
  $ hg annot mercurial/commands.py --time --line-number > /dev/null
  time: real 19.540 secs (user 19.460+0.000 sys 0.080+0.000)

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).