Allow changing MR source branchs
If a topic-based merge request has to be retargeted to a different branch (for instance because a release branch has been created since the original MR submission), the logical Mercurial way of doing it is to change the branch of the topic changesets that the MR refers to.
From the point of view of GitLab (the Rails app), these amended changesets are now in a new branch, and the source branch of the MR actually disappeared (see #134 (closed)). Hence we must update the source branch of the MR, an action that the UI does not currently permit.
See !40 (merged) for a typical example, the contribution there has been changed from heptapod-0-6-stable
to heptapod
, then to heptapod-0-7
.
All this business is confusing to users, we should maybe make that automatic, it would spare us a change in the UI, but I feel this could become quite dangerous for the general case. Example: one could change the target branch only for the last changeset of a stack. We don't want to change the MR in that case.