git-rev-parse: Allow a "zeroth" parent of a commit - the commit itself.

This sounds nonsensical, but it's useful to make sure that the result is
a commit.

For example, "git-rev-parse v2.6.12" will return the _tag_ object for
v2.6.12, but "git-rev-parse v2.6.12^0" will return the _commit_ object
associated with that tag (and v2.6.12^1 will return the first parent).

Also, since the "parent" code will actually parse the commit, this,
together with the "--verify" flag, will verify not only that the result
is a single SHA1, but will also have verified that it's a proper commit
that we can see.

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