lib-rebase: document exec_ in FAKE_LINES

Signed-off-by: Andrew Pimlott <andrew@pimlott.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
t7500: fix flipped actual/expect

Signed-off-by: Andrew Pimlott <andrew@pimlott.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>