[PATCH 2 of 2] profiling: allow loading profiling extension before everything else

[Jun Wu]

Excerpts from Gregory Szorc's message of 2017-05-24 08:57:28 -0700:
> Am I correct in assuming that Facebook is doing some custom profiling work
> again? If so, can it just live upstream?

This is mostly a side effect of me writing the radixlink index stuff.

Code is at [1] (with some followups). I think upstream wants C code, so C++
and Cython are blockers. I'm currently more focused in lfs, chg, index at
present so cannot prioritize porting the profiler for now.

[1]: https://bitbucket.org/facebook/hg-experimental/commits/d02bde5c791dfc18cf346eca4e163dc09347342d