rebase-helper-v0: Git for Windows, rebase -i beta testers' edition

This is an early beta test for the rebase--helper. This helper is a
builtin command whose primary role is to accelerate performance-critical
parts of interactive rebases, so that they are not all that painful in
particular on Windows.

Its secondary role, of course, is to open the door for a fully builtin
interactive rebase.

This version is very special in that its interactive rebase is
*supposed* to be slower than Git's current one: it executes both the
current shell script version as well as the rebase--helper version and
then cross-validates the results.

That means that beta testers need to be prepared to perform the exact
same conflict resolutions, rewordings and amends *twice*, and to make
sure that the exec commands are safe to be executed *twice*, too,
because every interactive rebase will be performed *twice*.

For an early glimpse at the performance to expect, set the environment
variable GIT_USE_REBASE_HELPER=true before running `git rebase -i`, and
do not try this without validating the results manually.