fix: add test to demonstrate how many times tools are executed
The current implementation wastes a lot of effort invoking fixer tools more
than once on the same file name+content. It is good to track changes in this
behavior.
Differential Revision: https://phab.mercurial-scm.org/D11279
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh