exthelper: add some semi-useful trace logs It'd be nice to make the trace functions a little better-named in the output, but I'm not sure how much better we can do without overhead. This at least lets you see if a single reposetup function is eating all the time or if it's spread over all of them. I needed this because Google's uber-extension has a long load time and I wasn't sure where the problem was. Differential Revision: https://phab.mercurial-scm.org/D6381

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