filter-branch: assume HEAD if no revision supplied
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 <firstname.lastname@example.org> Signed-off-by: Junio C Hamano <email@example.com>