Elegant subdirectory checkout of remote-tracking branch?

I'm trying to figure out the most efficient way to keep an up to
date `todo` branch checked out in Meta [1].  I've tried a few
things like:

  $ git submodule add -b refs/remotes/origin/todo --reference ./ -- ./ Meta

and:

  $ git clone --single-branch --branch refs/remotes/origin/todo ./ Meta

These fail because I can't use a remote tracking branch as a
source for the clone.  It should be possible to do:

  $ git clone --reference . --single-branch --branch todo git://git.kernel.org/pub/scm/git/git.git Meta

but that will require (I think) network access during a fetch.
Since I'm already fetching `origin` from the superproject, I
don't want to have to refetch them for the submodule (or whatever
Meta ends up being).  Here's what I think happens with a
submodule fetch:

1. Query the remote URL to dereference its current `todo` branch.
2. Check if we have that object in our local object share (which
   we should, due to --reference and a recent superproject
   fetch).
3. Fetch any missing objects from the remote URL.

I want to replace step 1 with:

1b. Query the superproject to dereference its current
    `origin/todo` branch.

and step 3 with:

3b. Access objects from the superproject directly (as with
    --reference / --shared).

Do I need to setup something like:

  [remote "origin"]
    url = ../.git
    fetch = +refs/remotes/origin/todo:refs/remotes/origin/todo

by hand, or is there an easier way?

I can, of course, clone a local `todo` branch if I've set one up in my
superproject.  However, then I'd have to update-ref that branch to
sync with origin/todo after every fetch (that updates origin/todo).
This could be handled with a `git fetch` wrapper, but… yuck :p.

Any suggestions for an elegant solution would be appreciated :).
Once we figure something out, I can write it up and stick it in
howto/maintain-git.txt.

Cheers,
Trevor

[1]: http://article.gmane.org/gmane.comp.version-control.git/144748

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On Fri, Mar 01, 2013 at 10:22:53AM -0500, W. Trevor King wrote:

These fail because I can't use a remote tracking branch as a
source for the clone.  It should be possible to do:

  $ git clone --reference . --single-branch --branch todo git://git.kernel.org/pub/scm/git/git.git Meta

but that will require (I think) network access during a fetch.

Yes, it will. Junio mentioned already that for him, "Meta" is really a
separate repository, and I think the simplest thing is to just treat it
that way (that's how I handle my personal "meta" branch).

But if you really want to save the extra network round trip during a
fetch, you can either:

  1. Just fetch from the surrounding repository using a custom refspec,
     like:

       git init Meta
       cd Meta
       git config add remote.origin.url ..
       git config add remote.origin.fetch \
         refs/remotes/origin/todo:refs/remotes/origin/todo
       git fetch
       git checkout todo

or

  2. Look into the git-new-workdir script in contrib/workdir, which lets
     you check out an alternate branch in a separate directory.

The latter would probably be the most seamless, but it's also the most
likely to have bugs. :)

-Peff