rebase -i: check for updated todo after squash and reword

rebase -i: check for updated todo after squash and reword

While a rebase is stopped for the user to edit a commit message it can
be convenient for them to also edit the todo list. The scripted version
of rebase supported this but the C version does not. We already check to
see if the todo list has been updated by an exec command so extend this
to rewords and squashes. It only costs a single stat call to do this so
it should not affect the speed of the rebase (especially as it has just
stopped for the user to edit a message)

Note that for squashes the editor may be opened on a different pick to
the squash itself as we edit the message at the end of a chain fixups
and squashes.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>