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though the ref is outdated, I would expect git-bundle(1) to just take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