bundle-uri: download bundles from an advertised list

bundle-uri: download bundles from an advertised list

The logic in fetch_bundle_uri() is useful for the --bundle-uri option of
'git clone', but is not helpful when the clone operation discovers a
list of URIs from the bundle-uri protocol v2 command. To actually
download and unbundle the advertised bundles, we need a different
mechanism.

Create the new fetch_bundle_list() method which is very similar to
fetch_bundle_uri() except that it relies on download_bundle_list()
instead of fetch_bundle_uri_internal(). The download_bundle_list()
method will recursively call fetch_bundle_uri_internal() if any of the
advertised URIs serve a bundle list instead of a bundle. This will also
follow the bundle.list.mode setting from the input list: "any" will
download only one such URI while "all" will download data from all of
the URIs.

In an identical way to fetch_bundle_uri(), the bundles are unbundled
after all of the bundle lists have been expanded and all necessary URIs.

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