Add mergeable API, add acceptance test, and fix bugs

Move MergeableInfo into gerrit-extension-api so we can add a method to
RevisionAPI returning it. Add an acceptance test exercising this new
API, which also exercises Mergeable.

Mergeable had never been exercised in acceptance tests, so this
flushed out some bugs. For one thing, we were passing nulls to the
MergeabilityCacheImpl.LoadHelper constructor, which checks for
non-nulls. For another, particularly galling, we failed to set the
mergeable field in the result of /mergeable in case of a cache hit.

Change-Id: Ib77d994b65af439e56ebc112f3000a71b4222c9b