.arcconfig

contrib: new script to read events from a named pipe and emit catapult traces

I'm starting to get more serious about getting some insight into where
we're spending our time, both in hg itself but also in the test
suite. As a first pass, I'm going to try and produce catapult
traces[0] that can be viewed with Chrome's `about:tracing` tool.

0: https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/edit#heading=h.nso4gcezn7n1

Differential Revision: https://phab.mercurial-scm.org/D4342

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