Search parent directories for default*.do.

Previously, we would only search for default*.do in the same directory in
the target; now we search parent directories as well.

Let's say we're in a/b/ and trying to build foo.o.  If we find
../../default.o.do, then we'll run

	cd ../..; sh default.o.do a/b/foo .o $TMPNAME

In other words, we still always chdir to the same directory as the .do file.
But now $1 might have a path in it, not just a basename.

t/defaults-nested/test.do

@@ -0,0 +1,36 @@
+exec >&2
+redo-ifchange \
+	file.x.y.z file.z file \
+	a/b/file.x.y.z a/b/file.y.z a/b/file.z a/b/file \
+	a/d/file.x.y.z a/d/file.y.z a/d/file.z a/d/file
+
+(cd a/b && redo-ifchange ../file.x.y.z ../file.y.z ../file.z ../file)
+
+check()
+{
+	if [ "$(cat $1)" != "$2" ]; then
+		echo "$1 should contain '$2'"
+		echo " ...got '$(cat $1)'"
+		exit 44
+	fi
+}
+
+check file.x.y.z "root file.x.y.z"
+check file.z "root file.z"
+check file "root file"
+
+check a/file.x.y.z "default.x.y.z file .x.y.z"
+check a/file.y.z "default.z file.y .z"
+check a/file.z "default.z file .z"
+check a/file "root a/file"
+
+check a/b/file.x.y.z "file file.x.y.z"
+check a/b/file.y.z "default.y.z file .y.z"
+check a/b/file.z "default.z b/file .z"
+check a/b/file "root a/b/file"
+
+check a/d/file.x.y.z "default file.x.y.z"
+check a/d/file.y.z "default file.y.z"
+check a/d/file.z "default file.z"
+check a/d/file "default file"
+