minimal/do: 'redo-ifchange' with no parameters shouldn't try to build all.

It should just build nothing.  Because sometimes you want to do something
like:
	redo-ifchange $(find -name '*.c')

And the find doesn't return any results.  This is consistent with what real
redo does.

Added a test to confirm that it works.

minimal/do

@@ -30,6 +30,7 @@ export REDO=$(cd "${dir:-.}" && echo "$PWD/$base")
 DO_TOP=
 if [ -z "$DO_BUILT" ]; then
 	DO_TOP=1
+	[ -n "$*" ] || set all  # only toplevel redo has a default target
 	export DO_BUILT=$PWD/.do_built
 	: >>"$DO_BUILT"
 	echo "Removing previously built files..." >&2
@@ -139,11 +140,7 @@ redo()
 
 
 set -e
-if [ -n "$*" ]; then
-	redo "$@"
-else
-	redo all
-fi
+redo "$@"
 
 if [ -n "$DO_TOP" ]; then
 	echo "Removing stamp files..." >&2