Race condition in git-bundle(1) create when ref is updated while running

Hi y'all,

I discovered a bug in git-bundle(1) create. There is a race condition happening when a ref gets updated while the bundle creation process is running.

To reproduce, I've been running git-bundle(1) with `create my.bndl --all --ignore-missing` in a debugger. I've set a breakpoint at bundle.c:515[1] where setup_revisions() is called. After stepping over this line I see in the debugger `revs.pending` is populated.

    (gdb) p *revs.pending.objects
    $6 = {item = 0x7a2fb0, name = 0x78d7e0 "refs/heads/master", path = 0x0, mode = 12288}
    (gdb) p *revs.pending.objects.item
    $7 = {parsed = 1, type = 1, flags = 0, oid = {hash = "R\026\370\365\304\b\236\302\234\344\232\372\024t4\302>\017\001c\000\000\000\000sS\344\367\377\177\000", algo = 1}}

The hash value is the binary representation of `5216f8f5c4089ec29ce49afa147434c23e0f0163`, the current HEAD of `master`. At this point I've updated `master` in another terminal window:

    git commit --allow-empty -m"dummy"

Then in the debugger I continue the process to create the bundle. The resulting bundle seems to be missing `refs/heads/master`.

I'm surprised this ref is completely omitted from the bundle. Even though the ref is outdated, I would expect git-bundle(1) to just take the old commit ID.

[1]: https://github.com/git/git/blob/5216f8f5c4089ec29ce49afa147434c23e0f0163/bundle.c#L515

-- Toon