$3 and stdout no longer refer to the same file.

This is slightly inelegant, as the old style
	echo foo
	echo blah
	chmod a+x $3

doesn't work anymore; the stuff you wrote to stdout didn't end up in $3.
You can rewrite it as:
	exec >$3
	echo foo
	echo blah
	chmod a+x $3

Anyway, it's better this way, because now we can tell the difference between
a zero-length $3 and a nonexistent one.  A .do script can thus produce
either one and we'll either delete the target or move the empty $3 to
replace it, whichever is right.

As a bonus, this simplifies our detection of whether you did something weird
with overlapping changes to stdout and $3.

t/.gitignore

@@ -11,3 +11,8 @@ test2.args
 /makedir
 /makedir.log
 /chdir1
+/silence
+/silence.do
+/touch1
+/touch1.do
+/deltest2

t/CC.do

@@ -1,3 +1,4 @@
+exec >$3
 cat <<-EOF
 	gcc -Wall -o /dev/fd/1 -c "\$1"
 EOF

t/LD.do

@@ -1,3 +1,4 @@
+exec >$3
 cat <<-EOF
 	OUT="\$1"
 	shift

t/clean.do

@@ -1,4 +1,5 @@
 redo example/clean curse/clean deps/clean "space dir/clean"
-rm -f c c.c c.c.c c.c.c.b c.c.c.b.b d mode1 makedir.log chdir1
-rm -f hello [by]ellow *.o *~ .*~ CC LD passfail
-rm -rf makedir
+rm -f c c.c c.c.c c.c.c.b c.c.c.b.b d mode1 makedir.log chdir1 \
+	hello [by]ellow *.o *~ .*~ CC LD passfail silence silence.do \
+	touch1 touch1.do
+rm -rf makedir

t/example/CC.do

@@ -1,5 +1,6 @@
 redo-ifchange config.sh
 . ./config.sh
+exec >$3
 cat <<-EOF
 	redo-ifchange \$1.c
 	gcc -MD -MF \$3.deps.tmp -o \$3 -c \$1.c

t/silencetest.do

@@ -0,0 +1,7 @@
+echo 'echo hello' >silence.do
+redo silence
+[ -e silence ] || exit 55
+echo 'true' >silence.do
+redo silence
+[ ! -e silence ] || exit 66
+rm -f silence.do

t/test.do

@@ -1,4 +1,5 @@
 redo-ifchange all
 ./hello >&2
 redo deltest deltest2 test.args test2.args passfailtest chdirtest \
-	curse/test deps/test "space dir/test" modetest makedir2
+	curse/test deps/test "space dir/test" modetest makedir2 \
+	silencetest touchtest

t/touchtest.do

@@ -0,0 +1,9 @@
+echo 'echo hello' >touch1.do
+redo touch1
+[ -e touch1 ] || exit 55
+rm -f touch1
+echo 'touch $3' >touch1.do
+redo touch1
+[ -e touch1 ] || exit 66
+[ -z "$(cat touch1)" ] || exit 77
+rm -f touch1.do