symmetric difference with --left-only vs. range notation

From: D. Ben Knoble <ben.knoble@gmail.com>
Date: Aug 13, 2025, 18:55 UTC
Message-ID: <CALnO6CBOWTCd-cREfGYL8DPPd=S-s3Y6s0cgwvBzCJZr6E6hvA@mail.gmail.com>
In-Reply-To: <10372e00-3bb1-453a-bb62-332aaa935b24@gmail.com>

On Sun, Aug 10, 2025 at 11:42 AM Phillip Wood <phillip.wood123@gmail.com> wrote:

Show 15 quoted lines

>
> Hi Julia
>
> On 09/08/2025 02:14, Julia Evans via GitGitGadget wrote:
> > From: Julia Evans <julia@jvns.ca>
> >
> > +Here is a more detailed description of what `git rebase <upstream>` does:
> > +
> > +First, it makes a list of all commits in the current branch that are not in
> > +`<upstream>`. This is the same set of commits that would be shown by `git log
> > +<upstream>..HEAD`.
>
> The existing text is not quite accurate here, it should really say `git
> log --cherry-pick --right-only <upstream>...HEAD` as we drop any commits
> from the branch that have already been cherry-picked to <upstream>.

This reminds me, I've been meaning to ask: is there a meaningful difference between the following 2 commands?

    git rev-list --count --left-only @{u}...
    git rev-list --count ..@{u}

In my estimation, since we're not using --cherry-pick here, the first says "list (count) the commits on upstream but not in HEAD, since the merge-base (which is reachable from both)" while the second says "list (count) the commits on upstream but not in HEAD." Is there ever a situation where those sets aren't the same?

Of course the symmetric difference is not always the same as the range notation, but when we add --left-only [or --right-only, by symmetry ;)], does this collapse to the 2-dot notation?

I've been assuming the latter is more performant, too, but if that's not the case, that's also good to know.