Git: Find the most recent common ancestor of two branches

git diff master...feature

shows all the new commits of your current (possibly multi-commit) feature branch.

man git-diff documents that:

git diff A...B

is the same as:

git diff $(git merge-base A B) B

but the ... is easier to type and remember.

As mentioned by Dave, the special case of HEAD can be omitted. So:

git diff master...HEAD

is the same as:

git diff master...

which is enough if the current branch is feature.

Finally, remember that order matters! Doing git diff feature...master will show changes that are on master not on feature.

I wish more git commands would support that syntax, but I don't think they do. And some even have different semantics for ...: What are the differences between double-dot ".." and triple-dot "..." in Git commit ranges?

You are looking for git merge-base. Usage:

$ git merge-base branch2 branch3
050dc022f3a65bdc78d97e2b1ac9b595a924c3f2