commitctx: extract _filecommit too This function is exclusively used in `commitctx`. So we should extract it too for consistency and to reduce the `localrepo` bloat. 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. Differential Revision: https://phab.mercurial-scm.org/D8710