dirstate: only call lstat once per flags invocation

This makes a big difference to performance in some cases.

hg --time locate 'set:symlink()'

mozilla-central (70,000 files):

  before: 2.92 sec
  after:  2.47

another repo (170,000 files):

  before: 7.87 sec
  after:  6.86

mercurial/dirstate.py

@@ -154,11 +154,14 b' class dirstate(object):'
     def flagfunc(self, buildfallback):
         if self._checklink and self._checkexec:
             def f(x):
-                p = self._join(x)
-                if os.path.islink(p):
-                    return 'l'
-                if util.isexec(p):
-                    return 'x'
+                try:
+                    st = os.lstat(self._join(x))
+                    if util.statislink(st):
+                        return 'l'
+                    if util.statisexec(st):
+                        return 'x'
+                except OSError:
+                    pass
                 return ''
             return f