Don't repack existing objects in fast-import

Some users of fast-import have been trying to use it to rewrite
commits and trees, an activity where the all of the relevant blobs
are already available from the existing packfiles.  In such a case
we don't want to repack a blob, even if the frontend application
has supplied us the raw data rather than a mark or a SHA-1 name.

I'm intentionally only checking the packfiles that existed when
fast-import started and am always ignoring all loose object files.

We ignore loose objects because fast-import tends to operate on a
very large number of objects in a very short timespan, and it is
usually creating new objects, not reusing existing ones.  In such
a situtation the majority of the objects will not be found in the
existing packfiles, nor will they be loose object files.  If the
frontend application really wants us to look at loose object files,
then they can just repack the repository before running fast-import.

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

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