RepoCommand: Add linkfile support.

RepoCommand: Add linkfile support.

Android wants them to work, and we're only interested in them for bare
repos, so add them just for that.

Make sure to use symlinks instead of just using the copyfile
implementation. Some scripts look up where they're actually located in
order to find related files, so they need the link back to their
project.

Change-Id: I929b69b2505f03036f69e25a55daf93842871f30
Signed-off-by: Dan Willemsen <dwillemsen@google.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jeff Gaston <jeffrygaston@google.com>
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>