files_initial_transaction_commit(): only unlock if locked

files_initial_transaction_commit(): only unlock if locked

Running git clone --single-branch --mirror -b TAGNAME previously
triggered the following error message:

	fatal: multiple updates for ref 'refs/tags/TAGNAME' not allowed.

This error condition is handled in files_initial_transaction_commit().

42c7f7ff9 ("commit_packed_refs(): remove call to `packed_refs_unlock()`", 2017-06-23)
introduced incorrect unlocking in the error path of this function,
which changes the error message to

	fatal: BUG: packed_refs_unlock() called when not locked

Move the call to packed_refs_unlock() above the "cleanup:" label
since the unlocking should only be done in the last error path.

Signed-off-by: Mathias Rav <m@git.strova.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>