Verify we know how to read a pack before trying to using it.

Verify we know how to read a pack before trying to using it.

If the pack format were to ever change or be extended in the future
there is no assurance that just because the pack file lives in
objects/pack and doesn't end in .idx that we can read and decompress
its contents properly.

If we encounter what we think is a pack file and it isn't or we don't
recognize its version then die and suggest to the user that they
upgrade to a newer version of GIT which can handle that pack file.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>