Overridden files were accidentally getting reclassified as static.

This is relatively harmless, since we treat them *almost* identically,
except that we print a warning for overridden files to remind you that
something fishy is going on.

Add a test for the actual warning message to ensure it is printed.  (I
don't like tests for specific warning messages, but it was necessary in
this case.)

t/350-deps/override/all.do

@@ -0,0 +1,34 @@
+exec >&2
+rm -f a b *.log stamp
+
+echo 1 >stamp
+redo b
+[ "$(cat b)" = "hello-a-1-b" ] || exit 11
+
+../../flush-cache
+echo 2 >stamp
+redo-ifchange b
+[ "$(cat b)" = "hello-a-1-b" ] || exit 21  # a unchanged; b not redone
+
+. ../../skip-if-minimal-do.sh
+
+# Unfortunately the test below depends on the specific wording of the
+# "override" warning message, of the form:
+#    redo: a - you modified it; skipping
+# That's because this is specifically a test that the warning message
+# gets generated.  I added that test because (of course) when we didn't
+# test it, the warning message accidentally got broken.  Oops.  If you
+# rephrase the message, you'll have to also change the test.
+
+../../flush-cache
+echo 3 >stamp
+echo over-a >a
+redo-ifchange b >$1.log 2>&1
+[ "$(cat b)" = "over-a-3-b" ] || exit 31  # a overwritten, b redone
+grep "a - " "$1.log" >/dev/null || exit 32  # expected a warning msg
+
+../../flush-cache
+echo 4 >stamp
+redo-ifchange b >$1.log 2>&1
+[ "$(cat b)" = "over-a-3-b" ] || exit 41  # a not changed, b not redone
+grep "a - " "$1.log" >/dev/null || exit 42  # still expect a warning msg