Remove unnecessary truncation of in-pack size during copy

The number of bytes to copy was truncated to an int, but the
pack's copyToStream() method expected to be passed a long here.
Pass through the long so we don't truncate a giant object.

Change-Id: I0786ad60a3a33f84d8746efe51f68d64e127c332
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>