Previously, the function decided if the repository is after the split
based on the OVN branch name. This didn't work when a commit hash or other
branch name was used.
This patch changes the function to look at the tree and decide if the given
commit that split the OVN from OVS is present in the given OVS branch.
Change-Id: I89e1f450e4db4c9554e368d75155bec137074ba6
dcd2e79c00