branch tracking: inherit upstream

At work, our primary SCM is perforce (p4), but many of us use git in front of that for day-to-day work. The build system requires information about what part of the P4 repository it is building. I've cobbled together changes to make this work with git, and have been using "git merge-base HEAD @{upstream}" to find the top-most git-p4 commit.

But @{upstream} is not automatically inherited by branches. It is fine when a local branch is a normal tracking branch of origin/master, like:

    $ git checkout master
    $ git rev-parse --symbolic-full-name @{upstream}
    refs/remotes/origin/master

But I'd also like this to work:

    $ git branch feature
    $ git checkout feature
    $ git rev-parse --symbolic-full-name @{upstream}
    error: No upstream branch found for ''
    @{upstream}
    error: No upstream branch found for ''
    fatal: ambiguous argument '@{upstream}': unknown revision or path not in the working tree.
    Use '--' to separate paths from revisions

I know I can do:

    $ git branch --set-upstream feature origin/master

but that is a pain. And I know I can track the local master, but that is not what I want.

I'm looking for something like "branch.autosetupupstream" that would cause the upstream of new branches to be copied from the old branch (when it exists). Does this make sense?

		-- Pete