Make sure quickfetch is not fooled with a previous, incomplete fetch.

This updates git-rev-list --objects to be a bit more careful
when listing a blob object to make sure the blob actually
exists, and uses it to make sure the quick-fetch optimization we
introduced earlier is not fooled by a previous incomplete fetch.

The quick-fetch optimization works by running this command:

	git rev-list --objects <<commit-list>> --not --all

where <<commit-list>> is a list of commits that we are going to
fetch from the other side.  If there is any object missing to
complete the <<commit-list>>, the rev-list would fail and die
(say, the commit was in our repository, but its tree wasn't --
then it will barf while trying to list the blobs the tree
contains because it cannot read that tree).

Usually we do not have the objects (otherwise why would we
fetching?), but in one important special case we do: when the
remote repository is used as an alternate object store
(i.e. pointed by .git/objects/info/alternates).  We could check
.git/objects/info/alternates to see if the remote we are
interacting with is one of them (or is used as an alternate,
recursively, by one of them), but that check is more cumbersome
than it is worth.

The above check however did not catch missing blob, because
object listing code did not read nor check blob objects, knowing
that blobs do not contain any further references to other
objects.  This commit fixes it with practically unmeasurable
overhead.

I've benched this with

	git rev-list --objects --all >/dev/null

in the kernel repository, with three different implementations
of the "check-blob".

 - Checking with has_sha1_file() has negligible (unmeasurable)
   performance penalty.

 - Checking with sha1_object_info() makes it somewhat slower,
   perhaps by 5%.

 - Checking with read_sha1_file() to cause a fully re-validation
   is prohibitively expensive (about 4 times as much runtime).

In my original patch, I had this as a command line option, but
the overhead is small enough that it is not really worth it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

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