exchange: introduce functionality to lock repos during each run
This introduces a exchange lock which writes a
.hg/scmperf-lock file. This
will help us in someway which I am still not sure.
The test failure seems unrelated.
I am not sure what trigger this (I remember a discussion where this came up, but I don't think we need anything special about that just yet).
Also, I won't think this locking scheme works because two different process could be trying to lock at the same time without them realising it.
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