TW-1688: task fails to import

- Thanks to Rainer Müller.

test/tw-1688.t

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+. bash_tap_tw.sh
+
+# TW-1688 task fails to import
+#   The problem is when a completed task, with a dependency is exported, then
+#   imported after the data is removed. On import, the circular dependency
+#   check didn't notice that a UUID failed to exist, and generated a JSON error.
+#
+#   Although an unusual circumstance, people do delete data from their
+#   completed.data file.
+
+task add one
+task log two depends:1
+
+JSON=$(mktemp /tmp/tw-1688.XXXXXXXXXX)
+task /two/ export > $JSON
+
+rm $TASKDATA/pending.data $TASKDATA/completed.data
+task import $JSON
+