When we're using copies() to find changes between the working directory and
its first parent for diff/status/etc., use dirstate.copies() directly.
This avoids doing a full statwalk for simple diffs (issue1090) and
removes a special case from the status command.
This can make a difference when there are filters involved and
decode(encode(working-dir-data)) != working-dir-data
even though
encode(decode(repo-data)) == repo-data
An example is a working dir file that uses only \n when you're using
the win32text extension.
We encode the previous state as a negative file size (AFAICS, previous
versions of hg always have size == 0 when state == 'r').
We save the state of 'm'erged and dirty files, because they're the
two states that indicate that a file has to be committed on a merge
to correctly record per-file history.
