The "non-cached" mode takes a different approach, and is
potentially the more useful of the two in that what it does can't
be emulated with a git write-tree + git diff-tree. Thus that's
the default mode. The non-cached version asks the question:
show me the differences between HEAD and the currently checked out
tree - index contents _and_ files that aren't up to date
which is obviously a very useful question too, since that tells
you what you could commit. Again, the output matches the git
diff-tree -r output to a tee, but with a twist.
The twist is that if some file doesn't match the index, we don't
have a backing store thing for it, and we use the magic
"all-zero" sha1 to show that. So let's say that you have edited
kernel/sched.c, but have not actually done a git update-index on
it yet - there is no "object" associated with the new state, and
you get:
torvalds@ppc970:~/v2.6/linux> git diff-index --abbrev HEAD
:100644 100664 7476bb... 000000... kernel/sched.c
i.e., it shows that the tree has changed, and that kernel/sched.c
is not up to date and may contain new stuff. The all-zero sha1
means that to get the real diff, you need to look at the object
in the working directory directly rather than do an
object-to-object diff.
Note
As with other commands of this type, git diff-index does not
actually look at the contents of the file at all. So maybe
kernel/sched.c hasn't actually changed, and it's just that
you touched it. In either case, it's a note that you need to
git update-index it to make the index be in sync.
Note
You can have a mixture of files show up as "has been updated"
and "is still dirty in the working directory" together. You
can always tell which file is in which state, since the "has
been updated" ones show a valid sha1, and the "not in sync
with the index" ones will always have the special all-zero
sha1.
