FindChangedPaths returns paths that are not changed from merge commit

Related Gitlab Issue: gitlab#23625 (closed)

When a user pushes a list of changes to gitlab we perform diff checks on the changes. If those changes include a merge commit the returned changed paths may also include changes that are already in history.

We can solve the gitlab issue linked above by using -c instead of -m in the FindChangedPaths git-diff-tree cmd. -c combines the diffs and only returns changes that are different in all parents e.g. conflict resolutions (where different from all parents) and other manual changes introduced during the merge.

I discussed this in slack with @jcaigitlab and @pks-gitlab.

@pks-gitlab pointed out that this would be a breaking change so rather than changing the flag we should introduce an enum to allow the client to specify how the output should be formatted.

Note: the output format for a merge commit when using -c is different to -m (each parent has a mode and sha and a status) so the nextPath function will need to return multiple gitalypb.ChangedPaths for each merge commit.