Re: [PATCH 2/3] git-bundle: die if a given ref is not included in bundle

Mark Levedahl [off-list ref] writes:

git>git bundle create t.bdl master --not master
Generating pack...
Done counting 0 objects.
Writing 0 objects.
Total 0 (delta 0), reused 0 (delta 0)
git>git ls-remote t.bdl
git>

e.g, an empty bundle is created without any error or warning. This is
the one case I believe an error should result: there is no use to
sending (or even creating) an empty bundle.

I agree that erroring on an empty output is a sensible _option_
just like pack-objects has --no-empty option.

The above is actually an interesting example in a different
sense.  When somebody did the following, what should be output?

	$ edit; git commit -a ;# on master
        $ git checkout -b side
        $ edit; git commit -a ;# on side
        $ git bundle create foo.bdl master side ^master
	$ git bundle verify foo.bdl

My answer is that it should list master and side as the
available heads and master itself as also a prerequisite (which
is not what the current code does).  

I think unbundling foo.bdl should be the moral equivalent of
fetching from the originating repository by somebody who has its
prerequisites as tips of some branches.  So I think foo.bdl
should list refs/heads/master as one of the available heads to
fetch/pull from, while requiring the same commit as prerequisite
of the bundle.  It is as if you tried "git fetch" and found out
that you are up to date.  Listing where the 'master' tip is,
even though you did not have to include any commits from that
branch, gives you a useful bit of information ("I am up to date
with respect to that branch").

So I think if you did this instead in the above sequence: 

	$ git bundle create foo.bdl master ^master

it would be sensible to have an option to error out because of
empty pack, but at the same time it would equally be sensible to
have an option to still create a bundle with an empty pack
contents.  In either case, the head and prerequisite section
should include the tip of the master.  Earlier, I said we should
error out if we do not find 'master' in the list of shown
objects, but I think it is more sensible to add it to both the
list of head _and_ prerequisites.