git-pickaxe: re-scan the blob after making progress with -C

git-pickaxe: re-scan the blob after making progress with -C

The reason to do this is the same as in the previous change for
line copy detection within the same file (-M).

Also this fixes -C and -C -C (aka find-copies-harder) logic; in
this application we are not interested in the similarity
matching diffcore-rename makes, because we are only interested
in scanning files that were modified, or in the case of -C -C,
scanning all files in the parent and we want to do that
ourselves.

Signed-off-by: Junio C Hamano <junkio@cox.net>