rebase -i: reread the todo list if `exec` touched it

rebase -i: reread the todo list if `exec` touched it

In the scripted version of the interactive rebase, there was no internal
representation of the todo list; it was re-read before every command.
That allowed the hack that an `exec` command could append (or even
completely rewrite) the todo list.

This hack was broken by the partial conversion of the interactive rebase
to C, and this patch reinstates it.

We also add a small test to verify that this fix does not regress in the
future.

Signed-off-by: Stephen Hicks <sdh@google.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>