merge-recursive: Small code clarification -- variable name and comments

merge-recursive: Small code clarification -- variable name and comments

process_renames() had a variable named "stage" and derived variables
src_other and dst_other whose purpose was not immediately obvious; also,
I want to extend the scope of this variable and use it later, so it should
have a more descriptive name.  Do so, and add a brief comment explaining
how it is used and what it relates to.

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