perf: add perfdirs command

This measures the cost of computing the dirstate's dirs structure,
which becomes very expensive in a large working directory.

contrib/perf.py

@@ -99,6 +99,15 b' def perfancestorset(ui, repo, revset):'
             rev in s
     timer(d)
 
+@command('perfdirs')
+def perfdirs(ui, repo):
+    dirstate = repo.dirstate
+    'a' in dirstate
+    def d():
+        dirstate.dirs()
+        del dirstate._dirs
+    timer(d)
+
 @command('perfdirstate')
 def perfdirstate(ui, repo):
     "a" in repo.dirstate