promisor-remote: introduce promisor.acceptFromServerUrl

promisor-remote: introduce promisor.acceptFromServerUrl

The "promisor-remote" protocol capability allows servers to advertise
promisor remotes, but doesn't allow these remotes to be automatically
configured on the client.

Let's introduce a new `promisor.acceptFromServerUrl` config variable
which contains a glob pattern, so that advertised remotes with a URL
matching that pattern will be automatically configured.

The glob pattern can optionally be prefixed with a remote name which
will be used as the name of the new local remote.

For now though, let's only introduce the functions to read and validate
the glob patterns and the optional prefixes.

Checking if the URLs of the advertised remotes match the glob patterns
and taking the appropriate action is left for a following commit.

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