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/overwrite3.do

@@ -0,0 +1,8 @@
+# we don't delete $3 here, we just truncate and overwrite it.  But redo
+# can detect this by checking the current file position of our stdout when
+# we exit, and making sure it equals either 0 or the file size.
+#
+# If it doesn't, then we accidentally wrote to *both* stdout and a separate
+# file, and we should get warned about it.
+echo hello world
+echo goodbye world >$3