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