promisor-remote: auto-configure unknown remotes

Previous commits have introduced the `promisor.acceptFromServerUrl`
config variable to allowlist some URLs advertised by a server through
the "promisor-remote" protocol capability.

However the new `promisor.acceptFromServerUrl` mechanism, like the old
`promisor.acceptFromServer` mechanism, still requires a remote to
already exist in the client's local configuration before it can be
accepted. This places a significant manual burden on users to
pre-configure these remotes, and creates friction for administrators
who have to troubleshoot or manually provision these setups for their
teams.

To eliminate this burden, let's automatically create a new `[remote]`
section in the client's config when a server advertises an unknown
remote whose URL matches a `promisor.acceptFromServerUrl` glob pattern.

Concretely, let's add four helpers:

 - sanitize_remote_name(): turn an arbitrary URL-derived string into a
   valid remote name by replacing non-alphanumeric characters,
   collapsing runs of '-', and prepending "promisor-auto-".

 - promisor_remote_name_from_url(): normalize the URL and extract
   host+port+path to build a human-readable base name, then pass it
   through sanitize_remote_name().

 - configure_auto_promisor_remote(): write the remote.*.url,
   remote.*.promisor and remote.*.advertisedAs keys to the repo
   config.

 - handle_matching_allowed_url(): pick the final name (user-supplied
   alias or auto-generated), handle collisions by appending "-1",
   "-2", etc., then call configure_auto_promisor_remote().

Let's also add should_accept_new_remote_url() which reuses the
url_matches_accept_list() helper introduced in a previous commit to
find a matching pattern, then delegates to handle_matching_allowed_url()
to create the remote.

And then let's call should_accept_new_remote_url() from the '!item'
(unknown remote) branch of should_accept_remote(), setting
`reload_config` so that the newly-written config is picked up.

Finally let's document all that by:

 - expanding the `promisor.acceptFromServerUrl` entry to describe
   auto-creation, the optional "name=" prefix syntax, the
   "promisor-auto-*" generation rules, and numeric-suffix collision
   handling, and by
 - adding a "remote.<name>.advertisedAs" entry to "remote.adoc".

Also let's extend the precedence paragraph added by a previous commit
to mention this new acceptance path: until now, the only way for
`promisor.acceptFromServerUrl` to trigger acceptance was to allow
field updates for a known remote. With this commit, it can also trigger
auto-creation of a previously-unknown remote whose advertised URL
matches the allowlist.

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

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