Fix and test handling of backslashes in hooks (#3909)

* Update hooks to use `read -r` and `printf`

This portably avoids any interpretation of backslash escapes by the
shell.

* Support running make_tc_task elsewhere

* Add a test for backslashes in task descriptions

test/test_hooks/on-add.dummy

@@ -2,9 +2,9 @@
 
 echo "on-add executed"
 
-while read TASK; do
+while read -r TASK; do
     echo "New task $TASK"
-    echo $TASK
+    printf "%s\n" "$TASK"
 done
 
 exit 0