[RFH] commit object name in git-diff-tree output header?

[Junio C Hamano <junkio@cox.net>]

This is a request for help from your clueless/forgetful
maintainer.

With a single commit as its parameter, 'git-diff-tree' outputs
the name of the commit object when run without the -v flag as
its first line, like this:

    $ git-diff-tree -p HEAD
    22a27042603fc9068ca562a8b5c46b3417f99777
    diff --git a/Documentation/git-apply.txt b/Documentation/git-apply.txt
    --- a/Documentation/git-apply.txt
    +++ b/Documentation/git-apply.txt
    @@ -9,7 +9,7 @@ git-apply - Apply patch on a GIT index f

     SYNOPSIS
     --------
     ...

I have a vague recollection to have seen some code that detects
this ^[0-9a-f]{40}$ pattern and uses it as a separator between
logical patchset, but I seem to be unable to find it now.

Does anybody have scripts that depend on this behaviour?  This
commit object name is carried over to format-patch output, and I
would like to change it now.  So if your own patch acceptance
script relies on this behaviour, please speak up before I break
it.

This has been the format since Jun 23, change made by Linus.
Before that change, we used to say two commit object ID, like
this:

    $ git-diff-tree -p HEAD
    aefa4a5... (from 5fca669...)
    diff --git a/apply.c b/apply.c
    --- a/apply.c
    +++ b/apply.c
    @@ -860,7 +860,6 @@ static int find_offset(const char *buf,
    ...