Usage: redo-whichdo target

Display .do search paths for specified target. Exits successfully
if the last .do in output if the found existing one.
Similar to apenwarr/redo:

    $ redo-whichdo x/y/a.b.o
    x/y/a.b.o.do
    x/y/default.b.o.do
    x/y/default.o.do
    x/y/default.do
    x/default.b.o.do
    x/default.o.do
    x/default.do
    default.b.o.do
    default.o.do
    default.do
    ../default.b.o.do
    ../default.o.do
    ../default.do
