git-pickaxe: cache one already found path per commit.

git-pickaxe: cache one already found path per commit.

Depending on how bushy the commit DAG is, this saves calls to
the internal diff-tree for fork-point commits.  For example,
annotating Makefile in the kernel repository saves about a third
of such diff-tree calls.

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