Fix corrupted large deltas

Fix corrupted large deltas

Large objects stored as deltas get unpacked by JGit into a loose
object, so they are cheaper to access later on.  This unpacking was
broken because TeeInputStream copied the wrong length into the loose
object, sometimes copying too many bytes into the result.  This
created a loose object that did not have the correct content, and
whose length did not match the length denoted in the object header.

Change-Id: I3ce1fd9f3dc5bd195249c7872b3bec49570424a2
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>