repopick: Support project paths detection on different branches

 * When some projects are declared in the manifests with specific
    changes (revision="refs/changes/../....../."), the path
    detection does not work, while most cases have a unique paths

 * Allow projects with unique branches to select their paths
    upon repopick with a warning about the selection

Change-Id: Ic873d69f57c78f233db3d0de4ebd529f896799ea