exthelper: reintroduce the ability to register filesets Same mechanism as revsets earlier in this series. The LFS extension is updated to provide test coverage.

exthelper: reintroduce the ability to register templates Same mechanism as revsets earlier in this series. The LFS extension is updated to provide test coverage. This also seems to make the test failure around issue6033 mentioned in 0a7f582f6f1f much less regular on Windows.

extensions: deprecate extsetup without a `ui` argument (API) 9.5 years should be enough time, but there were some tests for the old style still (which are now updated). Exthelper doesn't fallback to the old API, so this is for consistency. .. api:: The extension hook ``extsetup`` without a `ui` argument has been deprecated, and will be removed in the next version. Add a `ui` argument to avoid the deprecation warning.

largefiles: port revset registration to exthelper This tests the merge code that wasn't tested in the previous patch.