Assuming I'm on branch foo, I use git difftool main.
If I pull main to the latest, then that diff changes. However, if push up the PR to GitHub I see the diff I expect.
What is the command to say git difftool main-at-the-SHA-where-I-started-this-branch?
If you want to view locally the same diff as the one you see in your PR, you need to run :
git difftool main...foo # 3 dots, not a typo
The 3 dots notation for git diff is explained in the doc :
This form is to view the changes on the branch containing and up to the second , starting at a common ancestor of both .
git diff A...Bis equivalent togit diff $(git merge-base A B) B. You can omit any one of<commit>, which has the same effect as using HEAD instead.
It will show you the diff of your branch since it forked from main -- as opposed to the diff between your branch and the current state of main, which may change if other branches are merged.