tests: remove unused IgnoreTest exception AFAICT its last use was removed in d839e4820da7.

tests: use unittest.SkipTest unittest.SkipTest was introduced in Python 2.7. We previously defined it with our own class so we could run on Python 2.6.

changegroup: delete "if True" and reflow

changegroup: let callers pass in transaction to apply() (API) I think passing in the transaction makes it a little clearer and more consistent with bundle2.

repair: create transaction for bundle1 unbundling earlier See earlier patch for motivation.

unbundle: create transaction for bundle1 unbundling earlier See earlier patch for motivation.

exchange: create transaction for bundle1 unbundling earlier changegroup.apply() currently creates a transation if there isn't already one. Having the callers of that method pass in an existing transaction seems a little cleaner. To do that, we need to make sure all callers have a transaction. Since the transaction name is used as a hook argument (HG_TXNNAME), we need to match the name from changegroup.apply().