log: copy the way of ancestor traversal to --follow matcher (issue5376)
We can't use fctx.linkrev() because follow() revset tries hard to simulate
the traversal of changelog DAG, not filelog DAG. This patch fixes
_makefollowlogfilematcher() to walk file ancestors in the same way as
revset._follow().
I'll factor out a common function in future patches.
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh