BreakingChanges: early adopter option

BreakingChanges: early adopter option

Discussing the desire to make breaking changes, declaring that
breaking changes are made at a certain version boundary, and
recording these decisions in this document, are necessary but not
sufficient.  We need to make sure that we can implement, test, and
deploy such impactful changes.

Earlier we considered to guard the breaking changes with a run-time
check of the `feature.git<version>` configuration to allow brave
users and developers to opt into them as early adoptors.  But the
engineering cost to support such a run-time switch, covering new and
disappearing git subcommands and how "git help" would adjust the
documentation to the run-time switch, would be unrealistically high
to be worth it.

Formalize the mechanism based on a compile-time switch to allow
early adopters to opt into the breaking change in a version of Git
before the planned version for the breaking change.

Signed-off-by: Junio C Hamano <gitster@pobox.com>