Make sure the empty tree exists when needed in merge-recursive.

There are some baseless merge cases where git-merge-recursive will
try to compare one of the branches against the empty tree.  However
most projects won't have the empty tree object in their object database
as Git does not normally create empty tree objects.  If the empty tree
object is missing then the merge process will die, as it cannot load the
object from the database.  The error message may make the user think that
their database is corrupt when its actually not.

So instead we should just create the empty tree object whenever it is
needed.  If the object already exists as a loose object then no harm
done.  Otherwise that loose object will be pruned away later by either
git-prune or git-prune-packed.

Thanks goes to Junio for suggesting this fix.

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

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