filter-branch: return 2 when nothing to rewrite

filter-branch: return 2 when nothing to rewrite

Using the --state-branch option allows us to perform incremental filtering.
This may lead to having nothing to rewrite in subsequent filtering, so we need
a way to recognize this case.
So, let's exit with 2 instead of 1 when this "error" occurs.

Signed-off-by: Michele Locati <michele@locati.it>
Signed-off-by: Junio C Hamano <gitster@pobox.com>