Jeff King wrote:

quoted text
> Have you looked at the documentation for GIT_EXTERNAL_DIFF (try git(7))?
> I think it is a cleaner way of doing what you want (although I think you
> will get each file diffed individually, which is perhaps not what you
> want).
> 
> Something like:
> 
> $ cat >merge.sh <<EOF
> #!/bin/sh
> opendiff "" ""
> EOF
> $ GIT_EXTERNAL_DIFF=./merge.sh git-diff ...

I hadn't seen that, no, but that's not quite right.
(Wincent pointed out its flaws better than me. Basically,
opendiff is not really diff-like enough.)

And in any case, that launches Filemerge repeatedly
on every file separately, which makes reviewing a large diff
time-consuming and not very helpful.

quoted text
> write-tree? Yikes. If you want to diff against the working tree, then do
> that. If you want to diff against the index, then you probably want to
> git-checkout-index to a tmpdir, and diff against that.

Am I misunderstanding the documentation? From man git-write-tree

"Conceptually, git-write-tree sync()s the current index contents into a
  set of tree files. In order to have that match what is actually in your
  directory right now, you need to have done a git-update-index phase
  before you did the git-write-tree."

So git-write-tree precisely does give you the index not the working tree,
by my reading.

quoted text
>> git-archive --format=tar $OLD | (cd $TMPDIR1; tar xf -)
> 
> Again, this could be simpler and faster by using git-checkout-index
> (preceded by git-read-tree into a temp index, if you are comparing
> against a tree).

Erm, ok, this is rapidly approaching the limit of my git knowledge,
but while I can see git-read-tree will write a tree-ish into a temp
index,

(so presumably
git-read-tree --index-output=$TMPFILE <commit>
ought to work. Except it doesn't, I get the error message
fatal: unable to write new index file
),

I can't see how to make git-checkout-index read from a temp index.

And I'm assuming I don't want to go stomping all over the current
index just in order to do a diff, which shouldn't change the state
of my repository.

Is there a canonical way to checkout a given commit object into
a fresh directory?

-- 
Dr. Toby O. H. White
Dept. Earth Sciences,
Downing Street,
Cambridge CB2 3EQ
United Kingdom

Tel: +44 1223 333464
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