Update git2-rs

This brings in a commit which enables global template options by default as part
of initializing new git repositories, allowing a templates to be used as part of
`cargo new`.

Closes #498
auto merge of #596 : alexcrichton/cargo/update-git2, r=brson

This brings in a commit which enables global template options by default as part
of initializing new git repositories, allowing a templates to be used as part of
`cargo new`.

Closes #498