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
 
 if [ -n "$DO_TOP" ]; then
 	echo "Removing stamp files..." >&2

t/blank/blank.do

@@ -0,0 +1,2 @@
+redo-ifchange
+

t/test.do

@@ -4,4 +4,4 @@ redo nonshelltest shelltest \
 	deltest deltest2 test.args test2.args passfailtest chdirtest \
 	curse/test deps/test "space dir/test" modetest makedir2 \
 	silencetest touchtest stamp/test alwaystest ifcreate-test \
-	unicode
+	unicode blank/blank