Bump libgit2

[Compare these two pins](https://github.com/libgit2/libgit2/compare/054a795...783993101949040865ea933eb73e35df806249b0).

This brings in libgit2/libgit2#2269, libgit2/libgit2#2108, and libgit2/libgit2#2279 to fix issues with ignores, attributes, and index thread safety. These changes are important, but fairly significant undertakings.