Git::cat_blob: allow using an empty blob to fix git-svn breakage

Git::cat_blob: allow using an empty blob to fix git-svn breakage

Recent "git-svn optimization" series introduced Git::cat_blob() subroutine
whose interface was broken in that it returned the size of the blob but
signalled an error by returning 0.  You can never use an empty blob with
such an interface.

This fixes the interface to return a negative value to signal an error.

Reported by Björn Steinbrink.

Signed-off-by: Junio C Hamano <gitster@pobox.com>