perf: support obtaining contexts from perfrevset

Previously, perfrevset called repo.revs(), which only returns integer
revisions. Many revset consumers call repo.set(), which returns
changectx instances. Or they obtain a context manually later.

Since obtaining changectx instances when evaluating revsets is common,
this patch adds support for benchmarking this use case.

While we added an if conditional for every benchmark loop, it
doesn't appear to matter since revset evaluation dwarfs the cost
of a single if.

contrib/perf.py

@@ -463,9 +463,10 def perfrevlog(ui, repo, file_, **opts):
     fm.end()
 
 @command('perfrevset',
-         [('C', 'clear', False, 'clear volatile cache between each call.')]
+         [('C', 'clear', False, 'clear volatile cache between each call.'),
+          ('', 'contexts', False, 'obtain changectx for each revision')]
          + formatteropts, "REVSET")
-def perfrevset(ui, repo, expr, clear=False, **opts):
+def perfrevset(ui, repo, expr, clear=False, contexts=False, **opts):
     """benchmark the execution time of a revset
 
     Use the --clean option if need to evaluate the impact of build volatile
@@ -475,6 +476,9 def perfrevset(ui, repo, expr, clear=Fal
     def d():
         if clear:
             repo.invalidatevolatilesets()
+        if contexts:
+            for ctx in repo.set(expr): pass
+        else:
         for r in repo.revs(expr): pass
     timer(d)
     fm.end()