diffcore-rename: split locate_rename_dst into two functions

This function manages the mapping of destination pathnames
to filepairs, and it handles both insertion and lookup. This
makes the return value a bit confusing, as we return a newly
created entry (even though no caller cares), and have no
room to indicate to the caller that an entry already
existed.

Instead, let's break this up into two distinct functions,
both backed by a common binary search. The binary search
will use our normal "return the index if we found something,
or negative index minus one to show where it would have
gone" semantics.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>