Merge branch 'ab/rebase-in-c-escape-hatch'

The recently merged "rebase in C" has an escape hatch to use the
scripted version when necessary, but it hasn't been documented,
which has been corrected.

* ab/rebase-in-c-escape-hatch:
tests: add a special setup where rebase.useBuiltin is off
rebase doc: document rebase.useBuiltin

diff --cc builtin/rebase.c
Simple merge