RSpec tests harness: avoid useless Workhorse rebuilds
Since Workhorse upstream GitLab bundled Workhorse in the main
repository, the test harness is using it, with a detection system
to trigger rebuilds. Being based on Git, it wasn't working
properly, resulting in bin/rspec
always rebuilding Workhorse.
Here we adapt the rebuild triggering logic to Mercurial
One could expect a Makefile calling the Go compiler to run fast in any case, but that still took 14 seconds on my fast workstation, the irony being that the tests we are using in development of Heptapod are very seldom calling through Workhorse.