f: fix a Python 3 bytes/string issue

I suspect we should test this tool in isolation, but we don't yet. Oh well.

Differential Revision: https://phab.mercurial-scm.org/D5061

tests/f

@@ -88,9 +88,11 b' def visit(opts, filenames, outfile):'
             if opts.newer:
                 # mtime might be in whole seconds so newer file might be same
                 if stat.st_mtime >= os.stat(opts.newer).st_mtime:
-                    facts.append(b'newer than %s' % opts.newer)
+                    facts.append(b'newer than %s' % opts.newer.encode(
+                        'utf8', 'replace'))
                 else:
-                    facts.append(b'older than %s' % opts.newer)
+                    facts.append(b'older than %s' % opts.newer.encode(
+                        'utf8', 'replace'))
         if opts.md5 and content is not None:
             h = hashlib.md5(content)
             facts.append(b'md5=%s' % binascii.hexlify(h.digest())[:opts.bytes])