Backed out changeset cfa477946181824f4c769580ebf59710090ba08a

Only printenv was changed, not the actual execution of hooks.
And not setting an empty value might cause problems on platforms
which can't always remove variables from the environment.

doc/hgrc.5.txt

@@ -327,9 +327,10 b' hooks::'
   Also, hooks like "commit" will be called in all contexts that
   generate a commit (eg. tag) and not just the commit command.
 
-  Note2: Environment variables with empty values will not be passed to
-  hooks. For instance, $HG_PARENT2 will not be set for non-merge
-  changesets.
+  Note2: Environment variables with empty values may not be passed to
+  hooks on platforms like Windows. For instance, $HG_PARENT2 will
+  not be available under Windows for non-merge changesets while being
+  set to an empty value under Unix-like systems.
 
   The syntax for Python hooks is as follows: