merge-recursive: option to specify rename threshold

The recursive merge strategy turns on rename detection but leaves the
rename threshold at the default. Add a strategy option to allow the user
to specify a rename threshold to use.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge-recursive.h b/merge-recursive.h
index d21b446..54420f1 100644
--- a/merge-recursive.h
+++ b/merge-recursive.h
@@ -19,6 +19,7 @@
 	int verbosity;
 	int diff_rename_limit;
 	int merge_rename_limit;
+	int rename_score;
 	int call_depth;
 	struct strbuf obuf;
 	struct string_list current_file_set;