Correct usage of project.relpath for multi manifest workspaces.
Change-Id: Idc32873552fcdae6eec7b03dde2b2f31134b72fd
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/347534
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>