gitk: Make "find" on "Files" work again.
gitk: Make "find" on "Files" work again.

It was broken by the change to supply just the child id to
git-diff-tree rather than both child and parent.

Signed-off-by: Paul Mackerras <19a0ba370c443ba08d20b5061586430ab449ee8c@samba.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git