Make compatible with Gerrit stable-3.3 branch

In I2379e28b4 `ProjectLevelConfig`'s inheritence was changed to not
extend `VersionedMetaData`. As a result it no longer has a `.commit()`
method on it.

Instead, read the config directly from the git repo and manipulate it
there. This probably makes some subtle cache bugs go away as a side
effect, but it likely isn't important since it would only matter in a
high contention service user creation situation. Such a situation is
difficult to imagine.

Change-Id: I4c7c94dd4db189c4a5e818ac184ddb2c318ebcb4
3 files changed