Configuration Server fails calculating the common commit ancestor in case merge commits happened

[mariusoe]

In the current version, the configuration server will fail in certain scenarios when calculating the common ancestor of certain commits. This is required e,g. for getting the users which modified a file.

In the current logic, it is not handled if a commit is reachable from multiple child commits. Another problem is if no common ancestor exists which can happen (in version 1.12.1) due to an initial configuration sync using remote repositories.