merge-recursive: move the get_renames() function

merge-recursive: move the get_renames() function

Move this function so it can re-use some others (without either
moving all of them or adding an annoying split between function
declarations and definitions).  Cheat slightly by adding a blank line
for readability, and in order to silence checkpatch.pl.

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>