Fail if gitdir does not point to objdir during sync

There are a set of cases that can cause the git directory in
.repo/projects to point to a directory in .repo/project-objects that
is not the one specified in the manifest. This results in a tree that
is not sane, and so should cause a failure.

In order to reproduce the failure case:
1) Sync to any manifest
2) Change the 'name' of a project to a different repository. Leave the
   'path' the same.
3) Resync the modified project. The project-objects directory will not
   be created, and the projects directory will remain pointed at the old
   project-objects.

Change-Id: Ie6711b1c773508850c5c9f748a27ff72d65e2bf2