rebase: guard against missing files in read_basic_state()

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: artagnon

git-rebase.sh

@@ -84,6 +84,8 @@ keep_empty=
 test "$(git config --bool rebase.autosquash)" = "true" && autosquash=t
 
 read_basic_state () {
+	test -f "$state_dir/head-name" &&
+	test -f "$state_dir/onto" &&
 	head_name=$(cat "$state_dir"/head-name) &&
 	onto=$(cat "$state_dir"/onto) &&
 	# We always write to orig-head, but interactive rebase used to write to