[fix] 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/tests/buildout.txt

@@ -2805,10 +2805,10 @@ provide alternate locations, and even names for these directories::
     Creating directory '/sample-alt/work'.
     Creating directory '/sample-alt/developbasket'.
     Develop: '/sample-buildout/recipes'
-    Uninstalling d4.
-    Uninstalling d3.
     Uninstalling d2.
     Uninstalling debug.
+    Uninstalling d4.
+    Uninstalling d3.
 
     >>> ls(alt)
     d  basket

src/zc/buildout/tests/test_all.py

@@ -1697,7 +1697,7 @@ some evil recipes that exit uncleanly:
     >>> mkdir('recipes')
     >>> write('recipes', 'recipes.py',
     ... '''
-    ... import os
+    ... import sys
     ...
     ... class Clean:
     ...     def __init__(*_): pass
@@ -1705,10 +1705,10 @@ 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',
@@ -1802,10 +1802,10 @@ Now let's look at 3 cases:
 
     >>> print_(system(buildout+' buildout:parts='), end='')
     Develop: '/sample-buildout/recipes'
-    Uninstalling p2.
-    Uninstalling p1.
     Uninstalling p4.
     Uninstalling p3.
+    Uninstalling p2.
+    Uninstalling p1.
 
 3. We exit while installing or updating after uninstalling:
 

zc.recipe.egg_/src/zc/recipe/egg/custom.rst

@@ -434,8 +434,8 @@ Create a clean buildout.cfg w/o the checkenv recipe, and delete the recipe:
     ... """ % dict(server=link_server))
     >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
     Develop: '/sample-buildout/recipes'
-    Uninstalling checkenv.
     Uninstalling extdemo.
+    Uninstalling checkenv.
     Installing extdemo...
 
     >>> rmdir(sample_buildout, 'recipes')