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

Fix test `branch::tests::name_is_valid`

As per #721, some consumers may
prefer to build `libgit2` themselves to simplify distribution.

To test the vendored version, you can run:

```
cargo build --features vendored-libgit2`
```

That being said, the test `branch::tests::name_is_valid` fails on macOS. This
test is currently broken as per
#721 (comment)

* Add binding for git_repository_fetchhead_foreach

* Assert pointers are not null in fetchhead_foreach_cb

* Pass through remote url in bytes in `fetchhead_foreach`

* Document parameters for `fetchhead_foreach`

* Fix formatting of docs for `mergehead_foreach`

* Bump libgit2 submodule to 1.2.0

Follow the recent release of 1.2.0

* Update bindings