improved heuristic for --upstream-branch

git merge-base is what you want, I think, more specifically --upstream-branch should default to $(git merge-base HEAD upstream)

On Tue, Sep 14, 2010 at 03:17:08AM -0000, Launchpad Bug Tracker wrote:
> You have been subscribed to a public bug by Rolf Leggewie (r0lf):
>
> Binary package hint: git-buildpackage
>
> the current default for gbp and friends is to use HEAD of the upstream
> branch for --upstream-branch. Thinking about bug 615212, I felt that
> gbp could do much better than this. Consider the very simple commit
> graph in http://launchpadlibrarian.net/55567353/graph.txt for example.
> Let's say that commit X were to correspond to the lenny package, for
> example, and you want to make a security release or backport an
> important bugfix to stable. You branch off X and commit X'. When you
> now build against X' the default is to use the HEAD of upstream which is
> wrong and gbp has the necessary information to determine it is wrong.
>
> I think gbp should the find the "highest" common ancestor on the
> upstream branch and use that for --upstream-branch. I'm sure there is a
> ready-made, easy tool in git to do this.

Something like this is on the todo list since quiet some time, however
we can even do better and look at the specific tag of that upstream
version which so guessing is needed at all. This was delayed by #534494.

The corresponding Debian Bug is #564791.
 -- Guido