Git.pm: fix cat_blob crashes on large files

Git.pm: fix cat_blob crashes on large files

Read and write each 1024 byte buffer, rather than trying to buffer
the entire content of the file.  We are only copying the contents to
a file descriptor and do not use it ourselves.

Previous code would crash on all files > 2 Gib, when the offset
variable became negative (perhaps below the level of perl),
resulting in a crash.  On a 32 bit system, or a system with low
memory it might crash before reaching 2 GiB due to memory
exhaustion.

This code may leave a partial file behind in case of failure, where
the old code would leave a completely empty file.  Neither version
verifies the correctness of the content.  Calling code must take
care of verification and cleanup.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>