convert: allow the sink object to be wrapped when the extension isn't loaded The next patch will wrap the conversion code, in order to write out a requirement for 'lfs' when appropriate. Wrapping convcmd.convertsink() in an afterloaded callback works fine when the convert extension is enabled by the user. The problem here is that lfconvert uses the convert extension, whether or not it was formally enabled by the user. My first attempt was to have lfs install an afterloaded callback that would wrap the convert sink if convert was loaded, or wrap lfconvert if it wasn't. Then the lfconvert override could install an afterloaded callback to try wrapping the convert sink again, before calling the original lfconvert. But that breaks down if largefiles can't load the convert extension on the fly. [1] Further, some tests were failing with an error indicating that the size of the afterloaded list changed while iterating it. Yuya mentioned that maybe some bits of convert could be moved into core, but I'm not sure where to draw that line. The convertsink() method depends on the list of sinks, which in turn depends on the sink classes. [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-November/108038.html