$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/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