erp5_core: Fix unicode bug on ZODB History view.

Some objects become unicode when %s is applied and may cause
UnicodeDecodeError, thus use %r insead.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:39:35

passed ERP5.PerformanceTest-Master

00:27:34

failed ERP5.UnitTest-Master

01:18:11

failed ERP5.UnitTest-Master.Medusa

01:19:34

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:45

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:22