BreakingChanges: document removal of grafting

The grafting mechanism for objects has been deprecated in e650d06
(docs: mark info/grafts as outdated, 2014-03-05), which is more than a
decade ago. The mechanism can lead to hard-to-debug issues and has a
superior replacement with replace refs.

Follow through with the deprecation and mark grafts for removal in Git
3.0.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

Documentation/BreakingChanges.txt

@@ -102,6 +102,19 @@ Cf. <[email protected]>,
 
 === Removals
 
+* Support for grafting commits has long been superseded by git-replace(1).
+  Grafts are inferior to replacement refs:
++
+  ** Grafts are a local-only mechanism and cannot be shared across
+     repositories.
+  ** Grafts can lead to hard-to-diagnose problems when transferring objects
+     between repositories.
++
+The grafting mechanism has been marked as outdated since e650d0643b (docs: mark
+info/grafts as outdated, 2014-03-05) and will be removed.
++
+Cf. <[email protected]>.
+
 == Superseded features that will not be deprecated
 
 Some features have gained newer replacements that aim to improve the design in