R1:faf0a311ec9e

In Git repositories, use "git symbolic-ref HEAD ..." to select the default branch

Summary:
Depends on D20434. Fixes T5963. Broadly, the issue here is that when:

- You create a new, empty repository.
- Then, you work on some branch other than `master`, without ever creating `master`.

...you get a warning on `git clone`:

> warning: remote HEAD refers to nonexistent ref, unable to checkout

To fix this, point the symbolic-ref HEAD at `refs/heads/<default-branch>` after installing commit hooks.

This…