merge: move the filtering of ambiguous files to a dedicated function
I have multiple reasons:
- The body of `_update` is way too long
- This adds typing which will help our tooling and brains understand this
code more easily
- This function will get more nested and complex in the next patch
I've taken the liberty of rewrapping and typo-passing the docstring.
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh