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`.

discovery-helper: echo the stripped revsets early Having them printed early make it easy for a user to just grab the generated revset and directly uses them

contrib: move the `discovery-helper.sh` script in`perf-utils` directory The script appeared before the directory. However the directory exists for this kind of script. We move it there.

tests: stabilize test-split.t for Windows It looks like there will be additional problems beyond this trivial fix, but this should make the bot green again.

tests: stabilize test-share.t on Windows PYTHON was not getting mangled for MSYS style paths, and remote was spitting out remote: 'C' is not recognized as an internal or external command, remote: operable program or batch file. (once -q was removed). Additionally, this should fix a failure with py3 because of spaces in the path.

split: add tests which demonstrate the issue5864 Differential Revision: https://phab.mercurial-scm.org/D6057

discovery: clarify why the caching of children is valid Yuya Nishihara pointed out that the code looks wrong without this clarification. (And, unsurprisingly, Yuya is right)