Update libgit2 to 65f6c1c

https://github.com/libgit2/libgit2/compare/3f8d005a82b39c504220d65b6a6aa696c3b1a9c4...65f6c1c7eb7beb412e30cacd5cbb2b00e1e975b0
What happens when you have two app domains using libgit2sharp but neither is the default app domain?

Would making `git_libgit2_init` return the initialization state (ie, are you the first caller) help here?