Yet another base64 patch

I am assuming this will be the last one one way or another...

I decided that filenames/tags beginning with - was a really bad thing,
so I decided that, ugly though it might be, the best was to do a hybrid
between regular base64 (+ /) and filesystem-safe base64 (- _) and use
+ _ as the nonalpha characters needed.  I have updated the base64
patches as well as gitcvt, and also put out a flat version of gitcvt.

gitcvt also now converts the HEAD file over.  This requires pointing it
at the .dircache/.git directory instead of the objects directory inside.
  I have tested it on both the git and the kernel-test repositories.

Checking out the total kernel tree (time checkout-cache -a into an empty directory):

	Cache cold	Cache hot
stock	3:46.95		19.95
base64	5:56.20		23.74
flat	2:44.13		15.68

It seems that the flat format, at least on ext3 with dircache, is actually a major performance win, and that the second level loses quite a bit.

	-hpa