Rename git_threads_ to git_libgit2_

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.

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I wonder if we should add `#define git_threads_init git_libgit2_init` in the header, since this is going to break everything that wants to update (then again, trying to use master is going to break compilation as well), and it would force the users to actually look at this and hopefully see that this hasn't been initialized.