Feature Request: commits where $LOCAL and $REMOTE are from

We got ourselves in a bit of a pickle. We are trying to write a custom diff/merge tool for some of our files in source control, but to display a readable diff, we need to read other files, that may or may not have changed.

We think we can solve this problem with two more variables in the command line that tell us which commit hash to fetch the additional files from (yes, it can be the working tree; we'd like to know if it is).