bump libgit2 to 0.21.3 to patch around security issue

[robinwhittleton]

http://article.gmane.org/gmane.linux.kernel/1853266 looks pretty bad, but libgit2 0.21.3 seems to patch it. Can this be imported?

[apepper]

+1

More details see https://github.com/blog/1938-vulnerability-announced-update-your-git-clients

[jameschao]

I've submitted a pull request to the objective-git project for this rowanj/objective-git#2.

[rylanb]

+1