branch.c: use ref transaction for all ref updates

branch.c: use ref transaction for all ref updates

Change create_branch to use a ref transaction when creating the new branch.

This also fixes a race condition in the old code where two concurrent
create_branch could race since the lock_any_ref_for_update/write_ref_sha1
did not protect against the ref already existing. I.e. one thread could end up
overwriting a branch even if the forcing flag is false.

Signed-off-by: Ronnie Sahlberg <redacted>
Reviewed-by: Michael Haggerty <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>