Project.pm: decline to set up cross device alternates

Project.pm: decline to set up cross device alternates

If the objects directory to be included in the alternates file has
a different device number than the project's own objects directory
decline to set up any alternates for the project.

Garbage collection would not work properly in such a case.

Girocco itself would never cause this situation to occur, but an
administrator could.  Should this situation arise, a newly created
fork whose objects live on a different device from its parent
project will not get the "implicit clone" that a same-device fork
does.  Too bad.  That's much safer from a gc perspective than
writing out a cross-device alternates entry.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>