refs: don't invoke reference-transaction hook for reflogs

refs: don't invoke reference-transaction hook for reflogs

The reference-transaction hook is invoked whenever there is a reference
update being performed. For each state of the transaction, we iterate
over the updates present and pass this information to the hook.

The `ref_update` structure is used to hold these updates within a
`transaction`. We use the same structure for holding reflog updates too.
Which means that the reference transaction hook is also obtaining
information about a reflog update. This is a bug, since:

  - The hook is designed to work with reference updates and reflogs
  updates are different.
  - The hook doesn't have the required information to distinguish
  reference updates from reflog updates.

This is particularly evident when the default branch (pointed by HEAD)
is updated, we see that the hook also receives information about HEAD
being changed. In reality, we only add a reflog update for HEAD, while
HEAD's values remains the same.

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