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 <[email protected]>

Author: gitster

Documentation/BreakingChanges.txt

@@ -59,10 +59,29 @@ over time. If circumstances change, an earlier decision to deprecate or change
 something may need to be revisited from time to time. So do not take items on
 this list to mean "it is settled, do not waste our time bringing it up again".
 
+== Procedure
+
+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.
+Because such changes are expected to be numerous, and the design and
+implementation of them are expected to span over time, they have to
+be deployable trivially at such a version boundary.
+
+The breaking changes MUST be guarded with the a compile-time switch,
+WITH_BREAKING_CHANGES, to help this process.  When built with it,
+the resulting Git binary together with its documentation would
+behave as if these breaking changes slated for the next big version
+boundary are already in effect.  We may also want to have a CI job
+or two to exercise the work-in-progress version of Git with these
+breaking changes.
+
+
 == Git 3.0
 
 The following subsections document upcoming breaking changes for Git 3.0. There
-is no planned release date for this breaking version yet.
+is no planned release date for this breaking version yet.  The early
+adopter configuration used for changes for this release is `feature.git3`.
 
 Proposed changes and removals only include items which are "ready" to be done.
 In other words, this is not supposed to be a wishlist of features that should