The implementation looks good, I've left a couple of small comments on
the tests.

Thanks for all the help so far! Much appreciated!

One thought I had was whether we want a mode which recurses
so that if the upstream of topic2 is topic1 which has an upstream of
origin/main --forked=recurse origin/main would list topic1 and topic2.
So long as we don't think that is a sensible default we can add it in
the future if we want.

I don't understand this one, but if we safely do it later then that
sounds good considering we will soon be on v19.

quoted

+             git branch local-base &&
+             git branch --track local-one origin/one &&
+             git branch --track local-two origin/two &&
+             git branch --track local-foreign other/foreign &&
+             git branch --track local-onbase local-base &&
+
+             git checkout local-one &&
+             test_commit --no-tag local-one-work local-one.t &&
+             git checkout local-foreign &&
+             test_commit --no-tag local-foreign-work local-foreign.t &&
+             git checkout --detach

Why do we need a detached HEAD?

The '--delete-merged honours branch.<name>.deleteMerged=false' ' and
"branch -d still deletes a deleteMerged=false branch" ' tests need it,
but it's not necessary here, deleting.

quoted

+     git -C forked branch --forked "origin/*" --no-merged origin/one \
+             --format="%(refname:short)" >actual &&
+     echo local-one >expect &&
+     test_cmp expect actual
+'
+
+test_expect_success '--forked rejects unknown branch/pattern' '
+     test_must_fail git -C forked branch --forked nope 2>err &&
+     test_grep "not a valid branch or pattern" err
+'
+
+test_expect_success '--forked requires a value' '
+     test_must_fail git -C forked branch --forked 2>err &&
+     test_grep "requires a value" err
+'

It is a bit odd to have these two tests in the middle of the ones that
check the functionality works.

Good point, reordering.


Harald