Tests: Add failing on-modify hook

- An on-modify hook that reverts all changes (i.e. returns the original
  task) should result in no changes.  Currently the "modified:"
  attribute gets updated despite no other changes being present.

test/hooks.on-modify.t

@@ -155,6 +155,24 @@ class TestHooksOnModify(TestCase):
         logs = hook.get_logs()
         self.assertEqual(logs["output"]["msgs"][0], "FEEDBACK")
 
+    def test_onmodify_revert_changes(self):
+        """on-modify-revert - revert all user modifications."""
+        hookname = 'on-modify-revert'
+        self.t.hooks.add_default(hookname, log=True)
+
+        code, out, err = self.t("add foo")
+        before = self.t.export()
+        self.t.faketime("+5s")
+        code, out, err = self.t("1 modify bar")
+        after = self.t.export()
+
+        # There should be absolutely _no_ changes.
+        self.assertEqual(before, after)
+
+        hook = self.t.hooks[hookname]
+        hook.assertTriggeredCount(1)
+        hook.assertExitcode(0)
+
 if __name__ == "__main__":
     from simpletap import TAPTestRunner
     unittest.main(testRunner=TAPTestRunner())

test/test_hooks/on-modify-revert

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+# The on-modify event is triggered separately for each task modified. This hook
+# script can accept/reject the modification. Processing will continue.
+
+# Input:
+# - line of JSON for the original task
+# - line of JSON for the modified task, the diff being the modification
+read original_task
+read modified_task
+
+# Output:
+# - JSON, modified or unmodified.
+# - Optional feedback/error.
+echo $original_task
+
+# Status:
+# - 0:     JSON accepted, non-JSON is feedback.
+# - non-0: JSON ignored, non-JSON is error.
+exit 0