Is the are git diff command to compare the merge base of the current branch and the working copy?
If you current branch is not master, you can try in a bash shell:
git diff $(git merge-base --fork-point master)
If uses git merge-base with --fork-point:
git merge-base --fork-point <ref> [<commit>]
Find the point at which a branch (or any history that leads to <commit>) forked from another branch (or any reference) <ref>.
This does not just look for the common ancestor of the two commits, but also takes into account the reflog of <ref> to see if the history leading to <commit> forked from an earlier incarnation of the branch <ref>.