Allow upload pack filters

Partial clones is a new feature area in Git that is being worked on by @chriscool and others. Part of partial clones is allowing the client to filter what to clone, and what not to. This is a feature that would require the server side to explicitly allow it, else the client will get the following message:

$ git clone --filter="blob:limit=1m" git@gitlab.com:gitlab-com/www-gitlab-com.git                                                                                                                                                                                              
Cloning into 'www-gitlab-com'...
warning: filtering not recognized by server, ignoring

To allow the server to filter upon request from the client a config value needs to be set: uploadpack.allowFilter, on a global level. Per the docs:

If this option is set, upload-pack will support partial clone and partial fetch object filtering.

Note that this configuration variable is ignored if it is seen in the repository-level config (this is a safety measure against fetching from untrusted repositories).

Git 2.18 supports this feature, and it might be nice for CI to support it too, although there's been no plans for this as of yet.