promisor-remote: use string constants for 'name' and 'url' too

promisor-remote: use string constants for 'name' and 'url' too

A previous commit started to define `promisor_field_filter` and
`promisor_field_token`, and used them instead of the
"partialCloneFilter" and "token" string literals.

Let's do the same for "name" and "url" to avoid repeating them
several times and for consistency with the other fields.

For skipping "name=" or "url=" in advertisements, let's introduce
a skip_field_name_prefix() helper function to keep parsing clean
and easy to understand.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>