download: handle shared projects a bit better

If a manifest checksout a project multiple times, repo download isn't
able to accurately pick the right project.  We were just picking the
first result which could be a bit random for the user.  If we hit that
situation, check if the cwd is one of the projects, and if it isn't,
we emit an error and tell the user it's an ambiguous request.

Bug: https://crbug.com/gerrit/13070
Change-Id: Id1059b81330229126b48c7312569b37504808383
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298702


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>