Fwd: $LOCAL and $REMOTE swapped for rebase mergetool

Hi,

When using `git mergetool` with `git merge`, the variables $BASE, $REMOTE, $LOCAL and $MERGED behave as I would expect.

However, when using `git rebase` instead of `git merge`, $LOCAL and $REMOTE are swapped - instead of $LOCAL being the local content before the commit and $REMOTE being the content being rebased, it's the other way round. This doesn't seem consistent with the git-mergetool man page.

This causes 3-way diff tools to swap the meaning of what's on the left pane and what's on the right pane. I had to work around this with the new 4-way diff tool that I wrote (https://github.com/noamraph/meld, check it out!) since it assumes the diff between BASE and REMOTE should be applied to LOCAL, and this doesn't hold.

I think that at least the documentation should be updated. Or perhaps the behavior should change - this actually seems to me like a bug.

Cheers, Noam