.editorconfig

encoding: add fast path of from/toutf8b() for ASCII strings

See the previous patch for why.

The added test seems not making much sense because ASCII strings should
never contain "\xed" and be valid UTF-8.

  (with mercurial repo)
  $ export HGRCPATH=/dev/null HGPLAIN=
  $ hg log --time --config experimental.stabilization=all -Tjson > /dev/null

  (original)
  time: real 6.830 secs (user 6.740+0.000 sys 0.080+0.000)
  time: real 6.690 secs (user 6.650+0.000 sys 0.040+0.000)
  time: real 6.700 secs (user 6.640+0.000 sys 0.060+0.000)

  (fast jsonescape)
  time: real 5.630 secs (user 5.550+0.000 sys 0.070+0.000)
  time: real 5.700 secs (user 5.650+0.000 sys 0.050+0.000)
  time: real 5.690 secs (user 5.640+0.000 sys 0.050+0.000)

  (this patch)
  time: real 5.190 secs (user 5.120+0.000 sys 0.070+0.000)
  time: real 5.230 secs (user 5.170+0.000 sys 0.050+0.000)
  time: real 5.220 secs (user 5.150+0.000 sys 0.070+0.000)

# See http://EditorConfig.org for the specification

root = true

[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true

[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true