py3: use '%f' for floats instead of '%s'

I remember Yuya saying we need to use bytestr() or '%r' because '%s' was clever.
Not sure it applies to this or not.

Differential Revision: https://phab.mercurial-scm.org/D4894

mercurial/copies.py

@@ -212,7 +212,7 b' def _committedforwardcopies(a, b, match)'
                 dbg('debug.copies:          rename of: %s\n' % ofctx._path)
             cm[f] = ofctx.path()
         if debug:
-            dbg('debug.copies:          time: %s seconds\n'
+            dbg('debug.copies:          time: %f seconds\n'
                 % (util.timer() - start))
     return cm