How to see the changes between two commits without commits in-between?
Asking for the difference /between/ two commits without including the commits in-between makes little sense. Commits are just snapshots of the contents of the repository; asking for the difference between two necessarily includes them. So the question then is, what are you really looking for?
As William suggested, cherry-picking can give you the delta of a single commit rebased on top of another. That is:
$ git checkout 012345$ git cherry-pick -n abcdef$ git diff --cached
This takes commit 'abcdef', compares it to its immediate ancestor, then applies that difference on top of '012345'. This new difference is then shown - the only change is the context comes from '012345' rather than 'abcdef's immediate ancestor. Of course, you may get conflicts and etc, so it's not a very useful process in most cases.
If you're just interested in abcdef itself, you can do:
$ git log -u -1 abcdef
This compares abcdef to its immediate ancestor, alone, and is usually what you want.
And of course
$ git diff 012345..abcdef
gives you all differences between those two commits.
It would help to get a better idea of what you're trying to achieve - as I mentioned, asking for the difference between two commits without what's in between doesn't actually make sense.