dirstate: refactor granularity code, add a test

- rename option dirstate.granularity
- move option reading into .write()
- add a simple test

tests/test-race

@@ -0,0 +1,15 b''
+#!/bin/sh
+
+mkdir t
+cd t
+hg init
+echo a > a
+hg add a
+hg commit -m test
+
+# do we ever miss a sub-second change?
+for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20; do
+    hg co -qC 0
+    echo b > a
+    hg st
+done

tests/test-race.out

@@ -0,0 +1,20 b''
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a
+M a

mercurial/dirstate.py

@@ -66,12 +66,6 b' class dirstate(object):'
         elif name == '_checkexec':
             self._checkexec = util.checkexec(self._root)
             return self._checkexec
-        elif name == '_limit':
-            try:
-                self._limit = int(self._ui.config('ui', 'limit', 1))
-            except ValueError:
-                self._limit = 1
-            return self._limit
         else:
             raise AttributeError, name
 
@@ -342,10 +336,15 b' class dirstate(object):'
         if not self._dirty:
             return
         st = self._opener("dirstate", "w", atomictemp=True)
-        if self._limit > 0:
-            limit = util.fstat(st).st_mtime - self._limit
-        else:
-            limit = sys.maxint
+
+        try:
+            gran = int(self._ui.config('dirstate', 'granularity', 1))
+        except ValueError:
+            gran = 1
+        limit = sys.maxint
+        if gran > 0:
+            limit = util.fstat(st).st_mtime - gran
+
         cs = cStringIO.StringIO()
         copymap = self._copymap
         pack = struct.pack