status: change + back out == clean (API)
After backing out a change, so the file contents is equal to a
previous revision of itself, we currently report the status between
the two equal revisions as modified. This is because
context._buildstatus() reports any file whose new nodeid is not equal
to _newnode as modified. That magic nodeid is given only to files
added or modified in the working directory, so any file whose nodeid
has changed between two revisions will be reported as modified.
Fix by simply comparing the file contents for all cases where the
nodeid changed, whether they are in the working copy or committed.
Marking with (API) as it subtly changes the semantics of the method.