merge-recursive: New function to assist resolving renames in-core only

merge-recursive: New function to assist resolving renames in-core only

process_renames() and process_entry() have nearly identical code for
doing three-way file merging to resolve content changes.  Since we are
already deferring some of the current rename handling in order to better
handle D/F conflicts, it seems to make sense to defer content merging as
well and remove the (nearly) duplicated code sections for handling this
merging.

To facilitate this process, add a new update_stages_and_entry() function
which will map the higher stage index entries from two files involved in a
rename into the resulting rename destination's index entries, and update
the associated stage_data structure.

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