subcmds: display correct path multitree messages

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>

subcmds/sync.py

@@ -723,7 +723,7 @@ later is required to fix a server side protocol bug.
         # ...we'll let existing jobs finish, though.
         if not success:
           ret = False
-          err_results.append(project.relpath)
+          err_results.append(project.RelPath(local=opt.this_manifest_only))
           if opt.fail_fast:
             if pool:
               pool.close()