perf: avoid actual writing branch cache out correctly

Mercurial 2.5 (or 9b6ae29d4801) introduced "perfbranchmap" command,
and tried to avoid actual writing branch cache out by replacing
write() of branchcache class in branchmap.py with no-op function
(probably, for elimination of noisy and heavy file I/O factor).

But its implementation isn't correct, because 9b6ae29d4801 replaced
not branchmap.branchcache.write() but branchmap.write(). The latter
doesn't exist, even at that change.

To avoid actual writing branch cache out correctly, this patch
replaces branchmap.branchcache.write() with no-op function.

To detect mistake of replacement or change of API in the future
quickly, this patch uses safeattrsetter() instead of direct attribute
assignment. For similarity between replacements, this patch also
changes replacement of branchmap.read().

In this patch, replacement of read()/write() can run safely outside
"try" block, because two safeattrsetter() invocations ensure that
replacement doesn't cause exception.

FYI, the table below compares "base" filter wall time of perfbranchmap
on recent mozilla-central repo with each Mercurial version between
before and after this patch.

  ==== ========= =========
  ver  before    after
  ==== ========= =========
  2.5  18.492334 18.232455
  2.6  18.733858 18.156702
  2.7  18.245598 18.349210
  2.8  18.289070 18.528422
  2.9  17.572742 16.989655
  3.0  17.406953 17.615012
  3.1  17.228419 17.689805
  3.2  17.862961 17.718367
  3.3   2.632110  2.707960
  3.4   3.285683  3.272060
  3.5   3.370141  3.352176
  3.6   3.366939  3.242455
  3.7   3.300778  3.367328
  3.8   3.300132  3.267298
  3.9   3.418996  3.370265
  ==== ========= =========

IMHO, there is no serious overlooking performance regression.

contrib/perf.py

@@ -884,16 +884,17 b' def perfbranchmap(ui, repo, full=False, '
             repo.filtered(name).branchmap()
     # add unfiltered
     allfilters.append(None)
-    oldread = branchmap.read
-    oldwrite = branchmap.branchcache.write
+
+    branchcacheread = safeattrsetter(branchmap, 'read')
+    branchcachewrite = safeattrsetter(branchmap.branchcache, 'write')
+    branchcacheread.set(lambda repo: None)
+    branchcachewrite.set(lambda bc, repo: None)
     try:
-        branchmap.read = lambda repo: None
-        branchmap.write = lambda repo: None
         for name in allfilters:
             timer(getbranchmap(name), title=str(name))
     finally:
-        branchmap.read = oldread
-        branchmap.branchcache.write = oldwrite
+        branchcacheread.restore()
+        branchcachewrite.restore()
     fm.end()
 
 @command('perfloadmarkers')