perf: make perfmanifest and perfnodelookup work with revsets

They were using repo[rev], which only works with a single symbol
(e.g. "." or "my-bookmark"), not general revsets. Switch them to
scmutil.revsingle() so they can also be used with e.g. ".^".

I don't actually care about these commands, but I want to remove uses
of repo[<string>].

Differential Revision: https://phab.mercurial-scm.org/D3083

contrib/perf.py

@@ -767,7 +767,7 b' def perfmanifest(ui, repo, rev, **opts):'
 @command('perfchangeset', formatteropts)
 def perfchangeset(ui, repo, rev, **opts):
     timer, fm = gettimer(ui, opts)
-    n = repo[rev].node()
+    n = scmutil.revsingle(repo, rev).node()
     def d():
         repo.changelog.read(n)
         #repo.changelog._cache = None
@@ -853,7 +853,7 b' def perfnodelookup(ui, repo, rev, **opts'
     timer, fm = gettimer(ui, opts)
     import mercurial.revlog
     mercurial.revlog._prereadsize = 2**24 # disable lazy parser in old hg
-    n = repo[rev].node()
+    n = scmutil.revsingle(repo, rev).node()
     cl = mercurial.revlog.revlog(getsvfs(repo), "00changelog.i")
     def d():
         cl.rev(n)