filter-branch: use require_clean_work_tree

Filter-branch already requires that we have a clean work
tree before starting. However, it failed to refresh the
index before checking, which means it could be wrong in the
case of stat-dirtiness.

Instead of simply adding a call to refresh the index, let's
switch to using the require_clean_work_tree function
provided by git-sh-setup. It does exactly what we want, and
with fewer lines of code and more specific output messages.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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