Better logging for 'manual override' detection.

The first time we notice a file has been overridden, log the old and
new stamp data, which might give a hint about how this happened.

Currently if I do
    rm t/950-curse/countall
    while :; do redo -j10 t/950-curse/all --shuffle || break; done

it will end up complaining that countall has been overridden within
just a few runs, even though it definitely hasn't been.  There seems to
be someone reading a file stamp while someone else is redoing the
file, but I haven't found it yet.

builder.py

@@ -107,7 +107,10 @@ class BuildJob:
             newstamp != state.STAMP_MISSING and 
             (sf.stamp != newstamp or sf.is_override)):
                 state.warn_override(_nice(t))
-                sf.set_override()
+                if not sf.is_override:
+                    warn('%s - old: %r\n' % (_nice(t), sf.stamp))
+                    warn('%s - new: %r\n' % (_nice(t), newstamp))
+                    sf.set_override()
                 sf.set_checked()
                 sf.save()
                 return self._after2(0)