user-friendliness sanity checks: catch common mistakes regarding $1/$2/$3.

.do files should never modify $1, and should write to *either* $3 or stdout,
but not both.  If they write to both, it's probably because they forgot to
redirect stdout to stderr, a very easy mistake to make but a hard one to
detect.

Now redo detects it for you and prints an informative message.

t/deps/overwrite.do

@@ -0,0 +1,4 @@
+redo overwrite1 2>&1 && exit 55
+redo overwrite2 2>&1 && exit 56
+redo overwrite3 2>&1 && exit 57
+exit 0