commitctx: return a richer object from _prepare_files
Instead of returning a lot of different list, we introduce a rich object that
hold all the file related information. The unique object help with data
consistency and simply functions arguments and return.
In the rest of this series we will increase usage of this object to simplify
more code.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).