Correctly handle undo with multiple tasks (#3886)

The `std::stringstream::clear` method does not, in fact, clear the
string -- it just resets some internal flags. Assigning a new
stringstream to the variable is not the most efficient way to do this,
but it's the clearest.

test/undo.test.py

@@ -27,6 +27,7 @@
 
 import sys
 import os
+import re
 import unittest
 
 # Ensure python finds the local simpletap module
@@ -61,6 +62,25 @@ class TestUndo(TestCase):
         code, out, err = self.t("_get 1.status")
         self.assertEqual(out.strip(), "pending")
 
+    def test_modify_multiple_tasks(self):
+        """'add' then 'done' then 'undo'"""
+        self.t("add one")
+        self.t("add two")
+        self.t("add three")
+        self.t("rc.bulk=0 1,2,3 modify +sometag")
+        code, out, err = self.t("undo", input="y\n")
+        # This undo output should show one tag modification for each task, possibly with some
+        # modification-time updates if the modifications spanned a second boundary.
+        self.assertRegex(
+            out,
+            "\s+".join(
+                [
+                    r"""[0-9a-f-]{36} (Update property 'modified' from\s+'[0-9]+' to '[0-9]+'\s+)?Add tag 'sometag'\s+Add property 'tags' with value 'sometag'"""
+                ]
+                * 3
+            ),
+        )
+
     def test_undo_en_passant(self):
         """Verify that en-passant changes during undo are an error"""
         self.t("add one")