perfbranchmap: display 'unfiltered' for unfiltered performance

This is slightly clearer than "None" and will help with coming changes to select
the filter level we want timing for.

contrib/perf.py

@@ -1616,7 +1616,10 b' def perfbranchmap(ui, repo, full=False, '
     branchcachewrite.set(lambda bc, repo: None)
     try:
         for name in allfilters:
-            timer(getbranchmap(name), title=str(name))
+            printname = name
+            if name is None:
+                printname = 'unfiltered'
+            timer(getbranchmap(name), title=str(printname))
     finally:
         branchcacheread.restore()
         branchcachewrite.restore()