diffcore-break: save cnt_data for other phases

The "break" phase works by counting changes between two blobs with the same path. We do this by splitting the file into chunks (or lines for text oriented files) and then keeping a count of chunk hashes. The "rename" phase counts changes between blobs at two different paths. However, it uses the exact same set of chunk hashes (which are immutable for a given sha1). The rename phase can therefore use the same hash data as break. Unfortunately, we were throwing this data away after computing it in the break phase. This patch instead attaches it to the filespec and lets it live through the rename phase, working under the assumption that most of the time that breaks are being computed, renames will be too. We only do this optimization for files which have actually been broken, as those ones will be candidates for rename detection (and it is a time-space tradeoff, so we don't want to waste space keeping useless data). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>