Use .keep files in fast-import during processing.

Use .keep files in fast-import during processing.

Because fast-import automatically updates all references (heads
and tags) at the end of its run the repository is corrupt unless
the objects are available in the .git/objects/pack directory prior
to the refs being modified.  The easiest way to ensure that is true
is to move the packfile and its associated index directly into the
.git/objects/pack directory as soon as we have finished output to it.

But the only safe way to do this is to create the a temporary .keep
file for that pack, so we use the same tricks that index-pack uses
when its being invoked by receive-pack.

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