merge: improve inexact rename limit warning

merge: improve inexact rename limit warning

The warning is generated deep in the diffcore code, which
means that it will come first, followed possibly by a spew
of conflicts, making it hard to see.

Instead, let's have diffcore pass back the information about
how big the rename limit would needed to have been, and then
the caller can provide a more appropriate message (and at a
more appropriate time).

No refactoring of other non-merge callers is necessary,
because nobody else was even using the warn_on_rename_limit
feature.

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