libgit2-sys affected by memory corruption, denial of service, and arbitrary code execution in libgit2

Summary

The [libgit2](https://github.com/libgit2/libgit2/) project fixed three security issues in the 1.7.2 release. These issues are:

* The `git_revparse_single` function can potentially enter an infinite loop on a well-crafted input, potentially causing a Denial of Service. This function is exposed in the `git2` crate via the [`Repository::revparse_single`](https://docs.rs/git2/latest/git2/struct.Repository.html#method.revparse_single) method. * The `git_index_add` function may cause heap corruption and possibly lead to arbitrary code execution. This function is exposed in the `git2` crate via the [`Index::add`](https://docs.rs/git2/latest/git2/struct.Index.html#method.add) method. * The smart transport negotiation may experience an out-of-bounds read when a remote server did not advertise capabilities.

The `libgit2-sys` crate bundles libgit2, or optionally links to a system libgit2 library. In either case, versions of the libgit2 library less than 1.7.2 are vulnerable. The 0.16.2 release of `libgit2-sys` bundles the fixed version of 1.7.2, and requires a system libgit2 version of at least 1.7.2.

It is recommended that all users upgrade.