Make some syntax change to follow coding style.

`git_buf_copy_cstr` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

Provide a function that can check reference name validity but can also
signal when an error occurs.  Use the name "name_is_valid", which is
more suggestive of checking a given name, rather than "is_valid_name",
which suggests that the function checks the validity of the current
reference's name.

Move `git_reference__is_valid_name` to `git_reference__name_is_valid`,
which returns errors and sets an out boolean parameter.

The `signature` variable in `git_reference_rename` isn't initialized and
neither does `git_reference__log_signature` always do. So if the latter
function fails, we'll call `git_signature_free` on this unininitialized
variable.

Fix the issue by initializing the pointer with `NULL`.

Access HEAD via the refdb backends