perf: document perfheads

contrib/perf.py

@@ -535,6 +535,7 b' def clearcaches(cl):'
 
 @command(b'perfheads', formatteropts)
 def perfheads(ui, repo, **opts):
+    """benchmark the computation of a changelog heads"""
     opts = _byteskwargs(opts)
     timer, fm = gettimer(ui, opts)
     cl = repo.changelog

tests/test-contrib-perf.t

@@ -88,7 +88,7 b' perfstatus'
                  (no help text available)
    perffncachewrite
                  (no help text available)
-   perfheads     (no help text available)
+   perfheads     benchmark the computation of a changelog heads
    perfhelper-pathcopies
                  find statistic about potential parameters for the
                  'perftracecopies'