diff.renamelimit is a basic diff configuration

diff.renamelimit is a basic diff configuration

The configuration was added as a core option in 3299c6f (diff: make
default rename detection limit configurable., 2005-11-15), but 9ce392f
(Move diff.renamelimit out of default configuration., 2005-11-21)
separated diff-related stuff out of the core.

Up to that point it was Ok.

When we separated the Porcelain options out of the git_diff_config in
83ad63c (diff: do not use configuration magic at the core-level,
2006-07-08), we should have been more careful.

This mistake made diff-tree plumbing and git-show Porcelain to notice
different set of renames when the user explicitly asked for rename
detection.

Signed-off-by: Junio C Hamano <gitster@pobox.com>