Small git writing cleanup, some initial reading support

[Linus Torvalds]

Ok, so I lied. The "git reading support" is more like just a very
rough skeleton that parses the git object tree hierarchy. And by that
I mean that it literally parses *just* the hierarchy, it doesn't ever
actually look at the data in the actual data files.

But that hierarchy does mean that you can find all the dives and trips
and create the data structures.

Anyway, the first patch is a trivial cleanup on the writing side, and
can be applied independently. The second patch I'm sending out because
it's where I'm finishing for today. If you feel that it doesn't make
sense to apply yet, I understand. I'll keep it around.

              Linus
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