Make diary-european-date-forms elements mutually exclusive

* lisp/calendar/calendar.el (diary-european-date-forms): Make the
elements mutually exclusive (bug#13536).
1 job for master in 56 minutes and 45 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #2997
test-all

00:56:45

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1