CoC: update to version 2.0 + local changes

CoC: update to version 2.0 + local changes

Update the CoC added in 5cdf2301 (add a Code of Conduct document,
2019-09-24 from version 1.4 to version 2.0. This is the version found
at [1] with the following minor changes:

 - We preserve the change to the CoC in 3f9ef874a73 (CODE_OF_CONDUCT:
   mention individual project-leader emails, 2019-09-26)

 - We preserve the custom intro added in 5cdf2301d4a (add a Code of
   Conduct document, 2019-09-24)

This change intentionally preserves a warning emitted on "git diff
--check". It's better to make it easily diff-able with upstream than
to fix whitespace changes in our version while we're at it.

1. https://www.contributor-covenant.org/version/2/0/code_of_conduct/code_of_conduct.md

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Acked-by: Christian Couder <redacted>
Acked-by: Derrick Stolee <redacted>
Acked-by: Elijah Newren <redacted>
Acked-by: Johannes Schindelin <redacted>
Acked-by: Jonathan Nieder <redacted>
Acked-by: brian m. carlson <redacted>
Acked-by: Jeff King <redacted>
Acked-by: Taylor Blau <redacted>
Acked-by: Jonathan Tan <redacted>
Acked-by: Junio C Hamano <redacted>
Signed-off-by: Junio C Hamano <redacted>