mktree --batch: build more than one tree object

mktree --batch: build more than one tree object

This option works in a similar way to the '--batch' option of 'git cat-file'.
It enables creation of many tree objects with a single process.

The change was motivated by performance considerations in applications that
need to create many tree objects. A non-rigorous test showed tree creation
times improved from (roughly) 200ms to 50ms.

Signed-off-by: Josh Micich <josh.micich@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>