Detect unsynchronizable repositories on multiple cluster hosts

Summary:
Ref T12613. Currently, the SVNTEST and HGTEST repositories are improperly configured on secure. These repositories use VCS systems which do not support synchronization, so they can not be served from cluster services with multiple hosts.

However, I've incorrectly configured them the same way as all the Git repositories, which support synchronization. This causes about 50% of requests to randomly fail (when they reach the wrong host).

Detect this issue and warn the user that the configuration is not valid.

It should be exceptionally difficult for normal installs to run into this.

Test Plan:

• Mostly faked these conditions locally, verified that secure really has this configuration.
• I'll push this, verify that the issue is detected correctly in production, then fix the config which should resolve the intermittent issues with SVNTEST.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12613

Differential Revision: https://secure.phabricator.com/D17774