merge-recursive: Rename conflict_rename_rename*() for clarity

merge-recursive: Rename conflict_rename_rename*() for clarity

The names conflict_rename_rename and conflict_rename_rename_2 did not make
it clear what they were handling.  Since the first of these handles one
file being renamed in both branches to different files, while the latter
handles two different files being renamed to the same thing, add a little
'1to2' and '2to1' suffix on these and an explanatory comment to make their
intent clearer.

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