D808: config: use copy-on-write to improve copy performance

[quark (Jun Wu)]

quark added a comment.


  I'm also sad about this. But we have a strong desire to make commands like `status` able to compete with native code. So every 1ms counts. If we cannot deliver that performance, the team might eventually end up with a native-rewrite approach. I'll be even sadder if that happens.
  
  I'm still optimistic though, as you can see the recent `alias` change removes 30ms overhead. The next big thing will be config caching - if statuses are not changed, do not re-parse the config. That's another 30-60ms for chg overhead.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D808

To: quark, #hg-reviewers, mbthomas, durin42
Cc: durin42, mbthomas, mercurial-devel