t3034: add rename threshold tests

t3034: add rename threshold tests

10ae752 (merge-recursive: option to specify rename threshold,
2010-09-27) introduced this feature but did not include any tests.

The tests use the new option --find-renames, which replaces the then
introduced and now deprecated option --rename-threshold.

Also update name and description of t3032 for consistency:
"merge-recursive options" -> "merge-recursive space options"

Signed-off-by: Felipe Gonçalves Assis <felipegassis@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>