merge-recursive: Restructure showing how to chain more process_* functions

merge-recursive: Restructure showing how to chain more process_* functions

In 3734893 (merge-recursive: Fix D/F conflicts 2010-07-09),
process_df_entry() was added to process_renames() and process_entry() but
in a somewhat restrictive manner.  Modify the code slightly to make it
clearer how we could chain more such functions if necessary, and alter
process_df_entry() to handle such chaining.

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