perf: don't turn byte to string when formatting perfbranchmap

I am not sure why this `str` all is there is the first place.

Differential Revision: https://phab.mercurial-scm.org/D9751

contrib/perf.py

@@ -3484,7 +3484,7 def perfbranchmap(ui, repo, *filternames
             printname = name
             if name is None:
                 printname = b'unfiltered'
-            timer(getbranchmap(name), title=str(printname))
+            timer(getbranchmap(name), title=printname)
     finally:
         branchcacheread.restore()
         branchcachewrite.restore()