merge-recursive: Consolidate process_entry() and process_df_entry()

merge-recursive: Consolidate process_entry() and process_df_entry()

The whole point of adding process_df_entry() was to ensure that files of
D/F conflicts were processed after paths under the corresponding
directory.  However, given that the entries are in sorted order, all we
need to do is iterate through them in reverse order to achieve the same
effect.  That lets us remove some duplicated code, and lets us keep
track of one less thing as we read the code ("do we need to make sure
this is processed before process_df_entry() or do we need to defer it
until then?").

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