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.

commitctx: gather more preparation code within the lock context This is a small change that exist mostly for clarification. I am about to move a large amount of code in its own function. having all that code next to each other will make the next changeset clearer.

commitctx: move a special case about files earlier Same logic as a changeset a bit earlier, the `writefilecopymeta` section is more a post processing details so we move the conditional about `files` value closer to the rest of the code computing `files` value.