bundle-uri: allow relative URLs in bundle lists

bundle-uri: allow relative URLs in bundle lists

Bundle providers may want to distribute that data across multiple CDNs.
This might require a change in the base URI, all the way to the domain
name. If all bundles require an absolute URI in their 'uri' value, then
every push to a CDN would require altering the table of contents to
match the expected domain and exact location within it.

Allow a bundle list to specify a relative URI for the bundles. This URI
is based on where the client received the bundle list. For a list
provided in the 'bundle-uri' protocol v2 command, the Git remote URI is
the base URI. Otherwise, the bundle list was provided from an HTTP URI
not using the Git protocol, and that URI is the base URI. This allows
easier distribution of bundle data.

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