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>
diff --git a/git-legacy-rebase.sh b/git-legacy-rebase.sh
index 699fc4d..37db5a7 100755
--- a/git-legacy-rebase.sh
+++ b/git-legacy-rebase.sh
@@ -99,6 +99,8 @@
 true)	gpg_sign_opt=-S ;;
 *)	gpg_sign_opt= ;;
 esac
+test "$(git config --bool rebase.reschedulefailedexec)" = "true" &&
+reschedule_failed_exec=--reschedule-failed-exec
 . git-rebase--common
 
 read_basic_state () {