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.

redo-whichdo.py

@@ -2,9 +2,9 @@
 import sys, os
 
 import vars_init
-vars_init.init([])
+vars_init.init_no_state()
 
-import builder
+import paths
 from log import err
 
 if len(sys.argv[1:]) != 1:
@@ -13,7 +13,7 @@ if len(sys.argv[1:]) != 1:
 
 want = sys.argv[1]
 abswant = os.path.abspath(want)
-for dodir,dofile,basedir,basename,ext in builder.possible_do_files(abswant):
+for dodir,dofile,basedir,basename,ext in paths.possible_do_files(abswant):
     dopath = os.path.join('/', dodir, dofile)
     relpath = os.path.relpath(dopath, '.')
     exists = os.path.exists(dopath)