Fixed #71: when needed, write versions to file

Picked versions are stored for later printing/writing in the Installer
also when show-picked-versions is set to false but update-versions-file is set
to a file. Previously, show-picked-versions=false would also disable the versions
file updating.

src/zc/buildout/buildout.py

@@ -325,8 +325,8 @@ class Buildout(DictMixin):
         self.show_picked_versions = bool_option(options,
                                                 'show-picked-versions')
         self.update_versions_file = options['update-versions-file']
-        zc.buildout.easy_install.show_picked_versions(
-            self.show_picked_versions)
+        zc.buildout.easy_install.store_picked_versions(
+            self.show_picked_versions or self.update_versions_file)
 
         download_cache = options.get('download-cache')
         if download_cache:
@@ -996,8 +996,6 @@ class Buildout(DictMixin):
 
     def _print_picked_versions(self):
         Installer = zc.buildout.easy_install.Installer
-        if not self.show_picked_versions:
-            return
         if not Installer._picked_versions:
             # Don't print empty output.
             return
@@ -1016,11 +1014,13 @@ class Buildout(DictMixin):
 
         output.extend(required_output)
 
+        if self.show_picked_versions:
             print_("Versions had to be automatically picked.")
             print_("The following part definition lists the versions picked:")
             print_('\n'.join(output))
+
         if self.update_versions_file:
-            # Also write to the versions file.
+            # Write to the versions file.
             if os.path.exists(self.update_versions_file):
                 output[:1] = [
                     '',

src/zc/buildout/easy_install.py

@@ -129,7 +129,7 @@ class Installer:
     _prefer_final = True
     _use_dependency_links = True
     _allow_picked_versions = True
-    _show_picked_versions = False
+    _store_picked_versions = False
 
     def __init__(self,
                  dest=None,
@@ -734,10 +734,10 @@ def allow_picked_versions(setting=None):
         Installer._allow_picked_versions = bool(setting)
     return old
 
-def show_picked_versions(setting=None):
-    old = Installer._show_picked_versions
+def store_picked_versions(setting=None):
+    old = Installer._store_picked_versions
     if setting is not None:
-        Installer._show_picked_versions = bool(setting)
+        Installer._store_picked_versions = bool(setting)
     return old
 
 
@@ -1260,7 +1260,7 @@ class MissingDistribution(zc.buildout.UserError):
 
 def _log_requirement(ws, req):
     if (not logger.isEnabledFor(logging.DEBUG) and
-        not Installer._show_picked_versions):
+        not Installer._store_picked_versions):
         # Sorting the working set and iterating over it's requirements
         # is expensive, so short circuit the work if it won't even be
         # logged.  When profiling a simple buildout with 10 parts with