Merge pull request ProgrammingRust#9 from Horki/ch_21_sync_once

ch21: libgit2-safe; replace deprecated const ONCE_INIT

Author: jimblandy

libgit2-rs-safe/src/git/mod.rs

@@ -75,7 +75,7 @@ use std;
 use libc;
 
 fn ensure_initialized() {
- static ONCE: std::sync::Once = std::sync::ONCE_INIT;
+ static ONCE: std::sync::Once = std::sync::Once::new();
  ONCE.call_once(|| {
  unsafe {
  check(raw::git_libgit2_init())