Merge branch 'ab/drop-scripted-rebase'

Retire scripted "git rebase" implementation.

* ab/drop-scripted-rebase:
rebase: remove the rebase.useBuiltin setting

diff --cc Makefile
Simple merge

diff --cc builtin/rebase.c
Simple merge

diff --cc t/README
Simple merge

diff --cc t/t3400-rebase.sh
Simple merge