Added an intercept to Mercurial's capabilities command to remove bundle2.

Summary:
If Mercurial 3.4+ is used to host repositories in Phabricator, any clients using 3.5+ will receive an exception after the bundle is pushed up. Clients will also fail to update phases for changesets pushed up.

Before directly responding to mercurial clients with all capabilities, this change filters out the 'bundle2' capability so the client negotiates using a legacy bundle wire format instead.

Test Plan:
Server: Mercurial 3.5
Client: Mercurial 3.4

Test with both HTTP and SSH protocols:

1. Create a local commit on client
2. Push commit to server
3. Verify the client emits something like:

searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 1 changes to 1 files

Closes T9450

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T9450

Differential Revision: https://secure.phabricator.com/D14241