Enable the packed refs file format

Enable the packed refs file format

This actually "turns on" the packed ref file format, now that the
infrastructure to do so sanely exists (ie notably the change to make the
reference reading logic take refnames rather than pathnames to the loose
objects that no longer necessarily even exist).

In particular, when the ref lookup hits a refname that has no loose file
associated with it, it falls back on the packed-ref information. Also, the
ref-locking code, while still using a loose file for the locking itself
(and _creating_ a loose file for the new ref) no longer requires that the
old ref be in such an unpacked state.

Finally, this does a minimal hack to git-checkout.sh to rather than check
the ref-file directly, do a "git-rev-parse" on the "heads/$refname".
That's not really wonderful - we should rather really have a special
routine to verify the names as proper branch head names, but it is a
workable solution for now.

With this, I can literally do something like

	git pack-refs
	find .git/refs -type f -print0 | xargs -0 rm -f --

and the end result is a largely working repository (ie I've done two
commits - which creates _one_ unpacked ref file - done things like run
"gitk" and "git log" etc, and it all looks ok).

There are probably things missing, but I'm hoping that the missing things
are now of the "small and obvious" kind, and that somebody else might want
to start looking at this too. Hint hint ;)

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>