commitctx: extract all the file preparation logic in a new function
Before we actually start to create a new commit we have a large block of logic
that do the necessary file and manifest commit and that determine which files
are been affected by the commit (and how).
This is a complex process on its own. It return a "simple" output that can be
fed to the next step. The output itself is not that simple as we return a lot of
individual items (files, added, removed, ...). My next step (and actual goal for
this cleanup) will be to simplify the return by returning a richer object that
will be more suited for the variation of data we want to store.
After this changeset the `commitctx` is a collection of smaller function with
limited scope. The largest one is still `_filecommit` without about 100 lines of
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).