To correctly track the line ranges over several branches,
we must make sure that we have processed all children before
reaching the commit itself.
Thus we introduce a first pass in cmd_line_log that runs
prepare_revision_walk to achieve the topological ordering.
Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>