I have.  But I want to avoid a regression at any cost.  And your patch 
just looks to me like it could do that.

What kind of regression?

But it _has_ been already suggested that you could provide arguments for 
the existing msg-hook, which would not break anything

Sure it won't break anything, but it won't work either!  The existing 
message hook runs after the editing session -- I want the hook to 
introduce text that is merely a suggestion that the user can delete, or 
a template that the user needs to customize further.

since the hook does 
not get any argument yet, and therefore existing hooks would be 
unaffected.

How does adding a new hook affect existing hooks?

Also, the change would be non-intrusive, easy-to-review

Please.  That's ludicrous.

My patch is 3 lines of inserted code and 0 modified lines, checking one 
variable that is set once in builtin-commit.c (edit_message).  The 
documentation says that it runs whenever the editor runs except for -c, 
and launch_editor runs after the 3 lines of code I inserted.

Seeing how biased you are, I don't really know why I bothered answering you.

take so much time away from the bug-fixing that we want to do right now in 

That's the first sensible argument that I hear.

Paolo