dirstate: fix rebuild; add a test

If rebuild calls invalidate, it will just queue a (re)read of the dirstate
file, while what we really want is to empty the current state.

tests/test-rebuildstate

@@ -0,0 +1,24 b''
+#!/bin/sh
+# basic test for hg debugrebuildstate
+
+hg init repo
+cd repo
+
+touch foo bar
+hg ci -Am 'add foo bar'
+
+touch baz
+hg add baz
+hg rm bar
+
+echo '% state dump'
+hg debugstate | cut -b 1-16,35- | sort
+echo '% status'
+hg st -A
+
+hg debugrebuildstate
+echo '% state dump'
+hg debugstate | cut -b 1-16,35- | sort
+echo '% status'
+hg st -A
+

tests/test-rebuildstate.out

@@ -0,0 +1,17 b''
+adding bar
+adding foo
+% state dump
+a 644          0 baz
+n 644          0 foo
+r   0          0 bar
+% status
+A baz
+R bar
+C foo
+% state dump
+n 666         -1 bar
+n 666         -1 foo
+% status
+! bar
+? baz
+C foo

mercurial/dirstate.py

@@ -240,8 +240,14 b' class dirstate(object):'
                 self._ui.warn(_("not in dirstate: %s!\n") % f)
                 pass
 
+    def clear(self):
+        self._map = {}
+        self._copymap = {}
+        self._pl = [nullid, nullid]
+        self.markdirty()
+
     def rebuild(self, parent, files):
-        self.invalidate()
+        self.clear()
         for f in files:
             if files.execf(f):
                 self._map[f] = ('n', 0777, -1, 0)