rebase: do not continue when the todo list generation failed

This is a *really* long-standing bug. As a matter of fact, this bug has
been with us from the very beginning of `rebase -i`: 1b1dce4bae (Teach
rebase an interactive mode, 2007-06-25), where the output of `rev-list`
was piped to `sed` (and any failure of the `rev-list` process would go
completely undetected).

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

git-rebase--interactive.sh

@@ -893,7 +893,8 @@ fi
 if test t != "$preserve_merges"
 then
 	git rebase--helper --make-script ${keep_empty:+--keep-empty} \
-		$revisions ${restrict_revision+^$restrict_revision} >"$todo"
+		$revisions ${restrict_revision+^$restrict_revision} >"$todo" ||
+	die "$(gettext "Could not generate todo list")"
 else
 	format=$(git config --get rebase.instructionFormat)
 	# the 'rev-list .. | sed' requires %m to parse; the instruction requires %H to parse