perf: make perf::bundle compatible down to 5.2

A another small change to make it compatible with a wider set of revision.

I did not check compatibility in the python-2 territory yet.

contrib/perf.py

@@ -1065,7 +1065,15 b' def perfbundle(ui, repo, *revs, **opts):'
 
     For now, this only supports "none" compression.
     """
+    try:
     from mercurial import bundlecaches
+
+        parsebundlespec = bundlecaches.parsebundlespec
+    except ImportError:
+        from mercurial import exchange
+
+        parsebundlespec = exchange.parsebundlespec
+
     from mercurial import discovery
     from mercurial import bundle2