Bump libgit2 to bring in gitignore fixes

https://github.com/libgit2/libgit2/pull/2746 is pretty important, so let's grab it.

Note that [alot changed](https://github.com/libgit2/libgit2/compare/4bb6ffb6bb8ad9d49eb173350be965183cd09c96...19ae843937bed26ddf522a4eb00953c81b1a4561) in libgit2 in the meantime.