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.
Apparently emacs sets TERM=dumb in its tty simulator, so even though
isatty() returns true, we shouldn't use colour codes. (emacs is therefore
lame. But we knew that.)
