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/.gitignore

@@ -1,2 +1,4 @@
 t1a
 t2.count
+overwrite
+overwrite[123]

t/deps/clean.do

@@ -1 +1,2 @@
-rm -f *~ .*~ *.count t1a
+rm -f *~ .*~ *.count t1a overwrite overwrite[123]
+

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

t/deps/overwrite1.do

@@ -0,0 +1,2 @@
+# this shouldn't be allowed; we're supposed to write to $3, not $1
+echo >$1

t/deps/overwrite2.do

@@ -0,0 +1,5 @@
+# this shouldn't be allowed; stdout is connected to $3 already, so if we
+# replace it *and* write to stdout, we're probably confused.
+echo hello world
+rm -f $3
+echo goodbye world >$3

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

t/deps/test.do

@@ -1,3 +1,3 @@
-redo test1 test2 ifchange-fail
+redo test1 test2 ifchange-fail overwrite