Write .installed.cfg only once, in safe way and only if there's any change.

Also, updating a part does not put it anymore at the end of the list of
installed parts, that was making .installed.cfg too big.

src/zc/buildout/debugging.txt

@@ -85,6 +85,8 @@ supply some input:
       File "/zc/buildout/buildout.py", line 1352, in main
         getattr(buildout, command)(args)
       File "/zc/buildout/buildout.py", line 383, in install
+        self._install_parts(install_args)
+      File buildout.py", line 791, in _install_parts
         installed_files = self[part]._call(recipe.install)
       File "/zc/buildout/buildout.py", line 961, in _call
         return f()

src/zc/buildout/tests.py

@@ -1498,7 +1498,7 @@ some evil recipes that exit uncleanly:
     >>> mkdir('recipes')
     >>> write('recipes', 'recipes.py',
     ... '''
-    ... import os
+    ... import sys
     ...
     ... class Clean:
     ...     def __init__(*_): pass
@@ -1506,10 +1506,10 @@ some evil recipes that exit uncleanly:
     ...     def update(_): pass
     ...
     ... class EvilInstall(Clean):
-    ...     def install(_): os._exit(1)
+    ...     def install(_): sys.exit(1)
     ...
     ... class EvilUpdate(Clean):
-    ...     def update(_): os._exit(1)
+    ...     def update(_): sys.exit(1)
     ... ''')
 
     >>> write('recipes', 'setup.py',
@@ -1606,7 +1606,6 @@ Now let's look at 3 cases:
     Uninstalling p2.
     Uninstalling p1.
     Uninstalling p4.
-    Uninstalling p3.
 
 3. We exit while installing or updating after uninstalling:
 
@@ -1682,7 +1681,6 @@ Now let's look at 3 cases:
 
     >>> print_(system(buildout), end='')
     Develop: '/sample-buildout/recipes'
-    Uninstalling p1.
     Installing p1.
     Updating p2.
     Updating p3.
@@ -2742,6 +2740,73 @@ def increment_on_command_line():
       recipe='zc.buildout:debug'
     """
 
+def bug_664539_simple_buildout():
+  r"""
+  >>> write('buildout.cfg', '''
+  ... [buildout]
+  ... parts = escape
+  ...
+  ... [escape]
+  ... recipe = zc.buildout:debug
+  ... foo = $${nonexistent:option}
+  ... ''')
+
+  >>> print_(system(buildout), end='')
+  Installing escape.
+    foo='${nonexistent:option}'
+    recipe='zc.buildout:debug'
+  """
+
+def bug_664539_reference():
+  r"""
+  >>> write('buildout.cfg', '''
+  ... [buildout]
+  ... parts = escape
+  ...
+  ... [escape]
+  ... recipe = zc.buildout:debug
+  ... foo = ${:bar}
+  ... bar = $${nonexistent:option}
+  ... ''')
+
+  >>> print_(system(buildout), end='')
+  Installing escape.
+    bar='${nonexistent:option}'
+    foo='${nonexistent:option}'
+    recipe='zc.buildout:debug'
+  """
+
+def bug_664539_complex_buildout():
+  r"""
+  >>> write('buildout.cfg', '''
+  ... [buildout]
+  ... parts = escape
+  ...
+  ... [escape]
+  ... recipe = zc.buildout:debug
+  ... foo = ${level1:foo}
+  ...
+  ... [level1]
+  ... recipe = zc.buildout:debug
+  ... foo = ${level2:foo}
+  ...
+  ... [level2]
+  ... recipe = zc.buildout:debug
+  ... foo = $${nonexistent:option}
+  ... ''')
+
+  >>> print_(system(buildout), end='')
+  Installing level2.
+    foo='${nonexistent:option}'
+    recipe='zc.buildout:debug'
+  Installing level1.
+    foo='${nonexistent:option}'
+    recipe='zc.buildout:debug'
+  Installing escape.
+    foo='${nonexistent:option}'
+    recipe='zc.buildout:debug'
+  """
+
 def test_constrained_requirement():
     """
     zc.buildout.easy_install._constrained_requirement(constraint, requirement)