merge-recursive: Move process_entry's content merging into a function

merge-recursive: Move process_entry's content merging into a function

This move is in preparation for merge_content growing and being called from
multiple places in order to handle D/F conflicts.

I also snuck in a small change to the output in the case that the merged
content for the file matches the current file contents, to make it better
match (and thus more able to take over) how other merge_file() calls in
process_renames() are handled.

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