Detect when a .do script deletes its stdout tmp file.

This can happen if we create the .tmp file in the same directory as the
target, and the .do file first does "rm -rf" on that directory, then
re-creates it.  The result is that the stdout file is lost.

We'll make this a warning if the .do script *didn't* write to stdout
(so the loss is harmless, just weird), and an error if they *did* write
to stdout, which we can detect because we still have an open fd on the
file, so we can fstat() it.

t/202-del/.gitignore

@@ -1 +1,2 @@
 deltest2
+/destruct

t/202-del/all.do

@@ -1 +1 @@
-redo deltest deltest2
+redo deltest deltest2 deltest3

t/202-del/clean.do

@@ -1 +1,2 @@
+rm -rf destruct
 rm -f deltest2 *~ .*~

t/202-del/deltest3.do

@@ -0,0 +1,25 @@
+rm -rf destruct
+mkdir destruct
+cd destruct
+cat >destruct1.do <<-EOF
+	rm -f *.tmp
+	echo 'redir' >\$3
+EOF
+cat >destruct2.do <<-EOF
+	rm -f *.tmp
+	echo 'stdout'
+EOF
+
+# deleting unused stdout file is a warning only
+redo destruct1 2>destruct1.log || exit 11
+[ "$(cat destruct1)" = "redir" ] || exit 12
+
+# deleting *used* stdout file may be a fatal mistake,
+# but we won't enforce that, since some redo variants
+# might be more accepting or use different tmp file
+# algorithms.  So either the file should be correct,
+# or it should be missing.
+redo destruct2 2>destruct2.log || :
+if [ -e "destruct2" ]; then
+	[ "$(cat destruct2)" = "stdout" ] || exit 22
+fi