redo-whichdo: fix a bug where the last dir was checked twice, and add tests.

When we can't find a .do file, we walk all the way back to the root
directory.  When that happens, the root directory is actually searched
twice.  This is harmless (since a .do file doesn't exist there anyway)
but causes redo-whichdo to produce the wrong output.

Also, add a test, which I forgot to do when writing whichdo in the
first place.

To make the test work from the root directory, we need a way to
initialize redo without actually creating a .redo directory.  Add a
init_no_state() function for that purpose, and split the necessary path
functions into their own module so we can avoid importing builder.py.

t/260-whichdo/all.do

@@ -0,0 +1 @@
+redo exists nonexists