merge-recursive: Cleanup and consolidation of rename_conflict_info

merge-recursive: Cleanup and consolidation of rename_conflict_info

The consolidation of process_entry() and process_df_entry() allows us to
consolidate more code paths concerning rename conflicts, and to do
a few additional related cleanups.  It also means we are using
rename_df_conflict_info in some cases where there is no D/F conflict;
rename it to rename_conflict_info.

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