filter-branch previously took the first non-option argument as the name for
a new branch. Since
dfd05e38, it now takes a revision or a revision range
and modifies the current branch. Update to operate on HEAD by default to
conform with standard git interface practice.
Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>