If the branch is current, or present in less than half of the projects,
list which projects it is *in*.
Otherwise, correctly detect which projects (by relpath) it is not in.
Previously, the "not in" path would incorrectly list all projects.
Change-Id: Ia153856f577035a51f538b7bf5d3135b70c69d52
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/328199
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Xin Li <delphij@google.com>