validate_headref: use skip_prefix for symref parsing

validate_headref: use skip_prefix for symref parsing

Since the previous commit guarantees that our symref buffer
is NUL-terminated, we can just use skip_prefix() and friends
to parse it. This is shorter and saves us having to deal
with magic numbers and keeping the "len" counter up to date.

While we're at it, let's name the rather obscure "buf" to
"refname", since that is the thing we are parsing with it.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>