connect: add function to parse multiple v1 capability values

connect: add function to parse multiple v1 capability values

In a capability response, we can have multiple symref entries.  In the
future, we will also allow for multiple hash algorithms to be specified.
To avoid duplication, expand the parse_feature_value function to take an
optional offset where the parsing should begin next time.  Add a wrapper
function that allows us to query the next server feature value, and use
it in the existing symref parsing code.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>