refs: rename `refs_create_symref()` to `refs_update_symref()`

refs: rename `refs_create_symref()` to `refs_update_symref()`

The `refs_create_symref()` function is used to update/create a symref.
But it doesn't check the old target of the symref, if existing. It force
updates the symref. In this regard, the name `refs_create_symref()` is a
bit misleading. So let's rename it to `refs_update_symref()`. This is
akin to how 'git-update-ref(1)' also allows us to create apart from
update.

While we're here, rename the arguments in the function to clarify what
they actually signify and reduce confusion.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>