Update the git2 dependency

It turned out most of the methods in libgit2 don't actually require a Signature
structure, they're all mostly optional. This commit updates to this version of
libgit2 where the arguments are all optional.
auto merge of #465 : alexcrichton/cargo/update-git2, r=wycats

It turned out most of the methods in libgit2 don't actually require a Signature
structure, they're all mostly optional. This commit updates to this version of
libgit2 where the arguments are all optional.

Closes #463