Revert "PackWriter: Do not delta compress already packed objects"

Revert "PackWriter: Do not delta compress already packed objects"

This reverts commit 67b064fc9fa7418fab83957b4f4e4baf9c6e08be.

The "tiny optimization" introduced by 67b0 turns out to have a big
savings on wall-clock time when the object store is very slow (e.g.
the DHT support in JGit), but comes with a much bigger penalty in
space used by the output stream.  CGit packed with 67b0 enabled is
7 MiB larger than it should be (36 MiB rather than 28/29 MiB).  The
much bigger Linux kernel repository gained over 200 MiB, though some
of this may have been caused by a smaller window setting.

Revert this patch as PackWriter should be optimizing for space used
rather than time spent, since its primary use is network transfer, and
that isn't free.

Change-Id: I7413a9ef89762208159b4a1adc5a22a4c9245611
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>