It would be incorrect to print ../$1.do (we only use $1.do in the
*current* directly, not prefix dirs; we only use default*.do in
those). However, when we found ../default.do, we forgot to print it,
because of a silly logic error.
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.
