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>

perl/Git.pm

@@ -890,20 +890,22 @@ sub cat_blob {
 	my $size = $1;
 
 	my $blob;
-	my $bytesRead = 0;
+	my $bytesLeft = $size;
 
 	while (1) {
-		my $bytesLeft = $size - $bytesRead;
 		last unless $bytesLeft;
 
 		my $bytesToRead = $bytesLeft < 1024 ? $bytesLeft : 1024;
-		my $read = read($in, $blob, $bytesToRead, $bytesRead);
+		my $read = read($in, $blob, $bytesToRead);
 		unless (defined($read)) {
 			$self->_close_cat_blob();
 			throw Error::Simple("in pipe went bad");
 		}
-
-		$bytesRead += $read;
+		unless (print $fh $blob) {
+			$self->_close_cat_blob();
+			throw Error::Simple("couldn't write to passed in filehandle");
+		}
+		$bytesLeft -= $read;
 	}
 
 	# Skip past the trailing newline.
@@ -918,11 +920,6 @@ sub cat_blob {
 		throw Error::Simple("didn't find newline after blob");
 	}
 
-	unless (print $fh $blob) {
-		$self->_close_cat_blob();
-		throw Error::Simple("couldn't write to passed in filehandle");
-	}
-
 	return $size;
 }