git-pickaxe: optimize by avoiding repeated read_sha1_file().

git-pickaxe: optimize by avoiding repeated read_sha1_file().

It turns out that pickaxe reads the same blob repeatedly while
blame can reuse the blob already read for the parent when
handling a child commit when it's parent's turn to pass its
blame to the grandparent.  Have a cache in the origin structure
to keep the blob there, which will be garbage collected when the
origin loses the last reference to it.

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