rebase: add a config option to default to --reschedule-failed-exec

rebase: add a config option to default to --reschedule-failed-exec

It would be cumbersome to type out that option all the time, so let's
offer the convenience of a config setting: rebase.rescheduleFailedExec.

Besides, this opens the door to changing the default in a future version
of Git: it does make some sense to reschedule failed `exec` commands by
default (and if we could go back in time when the `exec` command was
invented, we probably would change that default right from the start).

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