transaction: include txnname in the hookargs dictionary There is no reason to not include the txnname alongside the txnid in all case. The python hooks already have them, so aligning the the shell hooks seems it could be useful in the future. (I don't have a strong opinion about this, we can also decide to never align the python and shell hooks and this and I'll drop this patch).

discovery-helper: reflect argument value in the name of the results It is common to create multiple pairs of repositories using different argument values. Recording the argument value in the results names has two main advantages: * It is easy to remember the value used to create a pair, * It get simpler to create multiple pair at the same time from the same source. Previously, running: `./discovery-helper.sh pypy 50 10` would create a `pypy-left` and `pypy-right` repository. Now it will create `pypy-50h-10d-left` and `pypy-50h-10d-right`.