git log takes zero or more commits as arguments, showing the history leading up to that commit. When no argument is given, HEAD is assumed. For your case, you want to supply the two branch heads you want to compare:
git log --graph --oneline currentbranch otherbranch
If it doesn’t display too much, you can simplify this by using
git log --graph --oneline --all
which acts as if you had specified every reference in .git/refs as the commits to display.