merge-recursive: Update conflict_rename_rename_1to2() call signature

To facilitate having this function called later using information stored
in a rename_df_conflict_info struct, accept a diff_filepair instead of a
rename.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>