It looks like origin is the same as mr-origin when the contributor
pushes to one's branch. But when a reviewer rebases through the Gitlab
"Rebase" button on web GUI, I got a fatal error:
> fatal: ambiguous argument 'origin/asalle/use-dynamics-flag': unknown
revision or path not in the working tree.
Possibly `origin` is then the remote of the person who rebased (it would
be weird, yet it's a fact the branch is not found). Let's go with this
assumption.
Fixes#950
`.gitlab/search-common-ancestor.sh`'s original authors are
Philip Withnall and Frederic Martinsons.
(Jehan/reviewer's note: script further improved by Asalle)
Jehan