magit-read-file-from-rev: expect non-nil REVISION

The argument is mandatory, and if it is nil then that
is a bug in the caller and should be fixed there.
magit-read-file-from-rev: cosmetics
magit-revision-files: return all files

Return all files in the revision, not
just those below the current directory.
magit-read-file-{from-rev,hist}: move definitions
magit-find-file-read-args: cosmetics
magit-read-file-trace: remove duplicated definition