Ensure ObjectReader used by PackWriter is released

The ObjectReader API demands that we release the reader when we are
done with it.  PackWriter contains a reader, which it uses for the
entire packing session.  Expose the release of the reader through
a release method on the writer.

This still doesn't address the RevWalk and TreeWalk users, who
don't correctly release their reader.  But its a small step in the
right direction.

Change-Id: I5cb0b5c1b432434a799fceb21b86479e09b84a0a
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
8 files changed