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/nonexists.do

@@ -0,0 +1,45 @@
+exec >&2
+
+# Testing the search path for non-existent do files is a little tricky.
+# We can't be sure where our current directory is, so we don't know how
+# far up the stack redo will need to search.
+#
+# To dodge the problem, let's "cd /" first so that we're testing a target
+# relative to a known location (the root directory).
+
+if [ -e '/default.do' -o \
+     -e '/default.z.do' -o \
+     -e '/default.y.z.do' ]; then
+    echo "Weird: /default.*.do exists; can't run this test."
+    exit 99
+fi
+
+# redo-whichdo *should* fail here, so don't abort the script for that.
+set +e
+a=$(cd / && redo-whichdo __nonexist/a/x.y.z)
+rv=$?
+set -e
+
+if [ "$rv" -eq 0 ]; then
+    echo "redo-whichdo should return nonzero for a missing .do file."
+    exit 10
+fi
+
+b=$(cat <<EOF
+__nonexist/a/x.y.z.do
+__nonexist/a/default.y.z.do
+__nonexist/a/default.z.do
+__nonexist/a/default.do
+__nonexist/default.y.z.do
+__nonexist/default.z.do
+__nonexist/default.do
+default.y.z.do
+default.z.do
+default.do
+EOF
+)
+
+if [ "$a" != "$b" ]; then
+    printf 'redo-whichdo mismatch.\n\ngot:\n%s\n\nexpected:\n%s\n' "$a" "$b"
+    exit 11
+fi