[PATCH 0/3] Somebody updated my branch tip underneath me.

With this series, I am taking hints from Linus and trying to
illustrate a problem, show an approach to its solution and code
minimally to get others interested enough to follow through.

[PATCH 1/3] Add BASE index extension.
[PATCH 2/3] update-index --{set,get}-base
[PATCH 3/3] Use BASE index extension in git-commit and git-merge.

The problem description and the strategy to solve it are in the
commit log message of [PATCH 3/3].  There I only talk about
git-push from elsewhere while we are looking the other way, but
the same situation can also happen when you use a lightweight
shared working tree (i.e. Julian phillips's git-new-workdir) and
make a commit on a branch in one working tree while the other
working tree has a checkout of the same branch.

Let's see who are motivated enough to bite.