rebase --am: ignore rebase.rescheduleFailedExec

rebase --am: ignore rebase.rescheduleFailedExec

The `exec` command is specific to the interactive backend, therefore it
does not make sense for non-interactive rebases to heed that config
setting.

We still want to error out if a non-interactive rebase is started with
`--reschedule-failed-exec`, of course.

Reported by Vas Sudanagunta via:
https://github.com/git/git/commit/969de3ff0e0#commitcomment-33257187

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