Update libgit2 to 19ae843

[Old diff](https://github.com/libgit2/libgit2/compare/3f8d005a82b39c504220d65b6a6aa696c3b1a9c4...65f6c1c7eb7beb412e30cacd5cbb2b00e1e975b0)
[New diff](https://github.com/libgit2/libgit2/compare/65f6c1c7eb7beb412e30cacd5cbb2b00e1e975b0...19ae843937bed26ddf522a4eb00953c81b1a4561)
[Combined diff](https://github.com/libgit2/libgit2/compare/3f8d005a82b39c504220d65b6a6aa696c3b1a9c4...19ae843937bed26ddf522a4eb00953c81b1a4561)