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