Rename detection: Avoid repeated filespec population

In diffcore_rename, we assume that the blob contents in the filespec aren't required anymore after estimate_similarity has been called and thus we free it. But estimate_similarity might return early when the file sizes differ too much. In that case, cnt_data is never set and the next call to estimate_similarity will populate the filespec again, eventually rereading the same blob over and over again. To fix that, we first get the blob sizes and only when the blob contents are actually required, and when cnt_data will be set, the full filespec is populated, once. Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>