Fix test branch::tests::name_is_valid

The `Branch::name_is_valid` function allocates memory in `libgit2`, so we need
to initialize the memory allocator first via `git_libgit2_init`.

Author: arxanas

src/branch.rs

@@ -30,6 +30,7 @@ impl<'repo> Branch<'repo> {
 
     /// Ensure the branch name is well-formed.
     pub fn name_is_valid(name: &str) -> Result<bool, Error> {
+        crate::init();
         let name = CString::new(name)?;
         let mut valid: libc::c_int = 0;
         unsafe {