commitctx: compute files added from _filecommit returns
When possible, lets avoid recomputing the same information again.
This is part of a larger refactoring/cleanup of the commitctx code to clarify
and augment the logic gathering metadata useful for copy tracing. The current
code is a tad too long and entangled to make such update easy. We start with
easy and small cleanup.
Differential Revision: https://phab.mercurial-scm.org/D8703