git cat-file: Fix memory leak in batch mode

git cat-file: Fix memory leak in batch mode

When run in batch mode, git cat-file never frees the memory for the blob
contents it is printing. This quickly adds up and causes git-svn to be
hardly usable for imports of large svn repos, because it uses cat-file in
batch mode and cat-file's memory usage easily reaches several hundred MB
without any good reason.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>