Use revision of project config instead of loading it from repo

The current revision of the refs/meta/config branch is available from
the project config and we don't need to load it from the repository
once more on every load of a document.

Change-Id: Id318cf945b1673d5777a80997124ad5b2cdc7283
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>