contrib: add a write microbenchmark to perf.py

I'm adding some performance logging to ui.write - this benchmark lets us
confirm that the cost of that logging is acceptably low.

At this point, the microbenchmark on Linux over SSH shows:

! wall 3.213560 comb 0.410000 user 0.350000 sys 0.060000 (best of 4)

while on the Mac locally, it shows:

! wall 0.342325 comb 0.180000 user 0.110000 sys 0.070000 (best of 20)

contrib/perf.py

@@ -1269,6 +1269,17 b' def perflrucache(ui, size=4, gets=10000,'
         timer(fn, title=title)
         fm.end()
 
+@command('perfwrite', formatteropts)
+def perfwrite(ui, repo, **opts):
+    """microbenchmark ui.write
+    """
+    timer, fm = gettimer(ui, opts)
+    def write():
+        for i in range(100000):
+            ui.write(('Testing write performance\n'))
+    timer(write)
+    fm.end()
+
 def uisetup(ui):
     if (util.safehasattr(cmdutil, 'openrevlog') and
         not util.safehasattr(commands, 'debugrevlogopts')):

tests/test-contrib-perf.t

@@ -109,6 +109,7 b' perfstatus'
    perfvolatilesets
                  benchmark the computation of various volatile set
    perfwalk      (no help text available)
+   perfwrite     microbenchmark ui.write
   
   (use 'hg help -v perfstatusext' to show built-in aliases and global options)
   $ hg perfaddremove