On the other hand, if you want to always cause an action before
running a git opeation locally, you do not have to have a hook.
You can just prepare such a message based on GNU ChangeLog and
then run git-commit with -F, both inside your wrapper.

I see two other possibilities:

1) Would you prefer allowing to run a command by setting commit.template 
to something starting with an exclamation mark, i.e. something like 
"!git diff --cached --name-status -r"?

2) The pre-commit could receive a file name on the command line.  If it 
creates that file, it would be used as a template for the commit 
message.  The default implementation of pre-commit could include, 
commented out, a reimplementation of prepare_log_message.  I like this 
the least.

3) Consider that the patch, as I implemented it, does not act solely on 
local data, because the index could be modified by the pre-commit hook. 
  This possibility is contemplated explicitly in builtin-commit.c:

         /*
          * Re-read the index as pre-commit hook could have updated it,
          * and write it out as a tree.
          */

Paolo