perf: omit copying from ui.ferr to ui.fout for Mercurial earlier than 1.9

Before this patch, referring ui.ferr prevents perf.py from measuring
performance with Mercurial earlier than 1.9 (or 4e1ccd4c2b6d), because
ui.ferr isn't available in such Mercurial, even though there are some
code paths for Mercurial earlier than 1.9 in perf.py.

For example, setting "_prereadsize" attribute in perfindex() and
perfnodelookup() is effective only with hg earlier than 1.8 (or
61c9bc3da402).

contrib/perf.py

@@ -137,7 +137,11 b' def gettimer(ui, opts=None):'
         opts = {}
     # redirect all to stderr
     ui = ui.copy()
-    ui.fout = ui.ferr
+    uifout = safeattrsetter(ui, 'fout', ignoremissing=True)
+    if uifout:
+        # for "historical portability":
+        # ui.fout/ferr have been available since 1.9 (or 4e1ccd4c2b6d)
+        uifout.set(ui.ferr)
 
     # get a formatter
     uiformatter = getattr(ui, 'formatter', None)