perf: add a docstring to `perfpathcopies`

This will help people to find this command.

contrib/perf.py

@@ -836,6 +836,7 b' def perfmergecalculate(ui, repo, rev, **'
 
 @command(b'perfpathcopies', [], b"REV REV")
 def perfpathcopies(ui, repo, rev1, rev2, **opts):
+    """benchmark the copy tracing logic"""
     opts = _byteskwargs(opts)
     timer, fm = gettimer(ui, opts)
     ctx1 = scmutil.revsingle(repo, rev1, rev1)

tests/test-contrib-perf.t

@@ -104,7 +104,7 b' perfstatus'
                  (no help text available)
    perfparents   (no help text available)
    perfpathcopies
-                 (no help text available)
+                 benchmark the copy tracing logic
    perfphases    benchmark phasesets computation
    perfphasesremote
                  benchmark time needed to analyse phases of the remote server