Implement blob ID validation in fast-import.

Implement blob ID validation in fast-import.

When accepting revision SHA1 IDs from the frontend verify the SHA1
actually refers to a blob and is known to exist.  Its an error
to use a SHA1 in a tree if the blob doesn't exist as this would
cause git-fsck-objects to report a missing blob should the pack get
closed without the blob being appended into it or a subsequent pack.
So right now we'll just ask that the frontend "pre-declare" any
blobs it wants to use in a tree before it can use them.

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