Post-1.5.4 stuff: builtin-checkout

I've got my builtin-checkout implementation at:

 git://iabervon.org/~barkalow/git.git builtin-checkout

This may be interesting to some other people at this point, despite it not being relevant to 1.5.4, because (a) it includes a slew of small preliminary patches to other stuff that might be useful for making other things builtin and (b) these changes reflect my debugging of my naive implementation of builtin-checkout, and may suggest things that might be wrong in existing builtins that do multiple operations in the same process.

I'll actually send patches to the list post-1.5.4 of all this stuff.

	-Daniel
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