Refresh git configuration before looking for the tracking branch.

After publishing a new branch I noticed that `-trackingBranchWithError: success:` was still returning a `nil`, after a bit of investigation it turns out libgit2 was using a stale version of the repo's config when it was looking up the `branch.*.remote` and `branch.*.merge` in `git_branch_upstream_name`.

So I've added a `[configuration refresh:]` call before we call `git_branch_upstream`.
Compiler says no. Looks like it takes a `const` reference in the block. Although I could cast that error away, but at that point I'd prefer to use the `__block` qualifier.