Upgrade to the latest version of `libgit2`

[rafeca]

During the last upgrade of libgit2 (link), we couldn't update to a stable version because we needed a performance fix that was only available in the v0.28 branch but both the v0.28.0 and v0.28.1 versions had an issue that prevented us from upgrading to a stable version.

So we decided to pin libgit2 to a specific commit in its master branch (libgit2/libgit2@9084712).

Now that the issue in the v0.28 has been fixed, whenever a new stable release of libgit2 gets published we can upgrade to it.

Note: In order to do the upgrade, we should follow the instructions written in atom/atom#19007