refs: extract out refname verification in transactions

refs: extract out refname verification in transactions

Unless the `REF_SKIP_REFNAME_VERIFICATION` flag is set for an update,
the refname of the update is verified for:

  - Ensuring it is not a pseudoref.
  - Checking the refname format.

These checks will also be needed in a following commit where the
function to add reflog updates to the transaction is introduced. Extract
the code out into a new static function.

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