bundle: unbundle promisor packs

bundle: unbundle promisor packs

In order to have a valid pack-file after unbundling a bundle that has
the 'filter' capability, we need to generate a .promisor file. The
bundle does not promise _where_ the objects can be found, but we can
expect that these bundles will be unbundled in repositories with
appropriate promisor remotes that can find those missing objects.

Use the 'git index-pack --promisor=<message>' option to create this
.promisor file. Add "from-bundle" as the message to help anyone diagnose
issues with these promisor packs.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>