merge-recursive: Record more data needed for merging with dual renames

merge-recursive: Record more data needed for merging with dual renames

When two different files are renamed to one, we need to be able to do
three-way merges for both of those files.  To do that, we need to record
the sha1sum of the (possibly modified) file on the unrenamed side.  Modify
setup_rename_conflict_info() to take this extra information and record it
when the rename_type is RENAME_TWO_FILES_TO_ONE.

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