Add doNotDelta flag to ObjectToPack

Add doNotDelta flag to ObjectToPack

This flag will later control whether or not PackWriter search for a
delta base for this object.  Edge objects will never get searched,
as the writer won't be outputting them, so they should always have
this flag set on.  Sometime in the future this flag should also be
set for file blobs on file paths that have the "-delta" gitattribute
set in the repository's attributes file.

Change-Id: I6e518e1a6996c8ce00b523727f1b605e400e82c6
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>