Can you diff a working copy file versus another version that is not the previous version?

[Don Robbins]

With sourcetree it seems that you can only diff a working copy of a file versus its previous version. Is there a way to diff the working copy to any version of the same file without having to commit the file?

[Seth]

If I use "External Diff" by right-clicking a file in any commit, it pulls up a 3-way merge window with my working copy, a copy from the commit prior to the selected commit, and a copy from the selected commit.

I'm using SourceTree on Windows. My diff tool is meld with the paramaters "$BASE $REMOTE $LOCAL".

[Don Robbins]

In this case I am talking about files that have not been committed yet. These are files that have been modified and might be staged or unstaged. I would like to diff that file versus any other version like you can when you run log selected without having to commit the file first. 

[Seth]

Yes, that is what I mean by "working copy" as well. It's whatever file is in my folder structure in whatever state it is in. For a custom diff command, it appears that $BASE corresponds to the working copy of a file. $BASE may not be used for the pre-configured diff tools, I'm not sure.