Fix one case of missing object

Fix one case of missing object

When a repository is being GCed and a concurrent push is received, there
is the possibility of having a missing object. This is due to the fact
that after the list of objects to delete is built, there is a window of
time when an unreferenced and ready to delete object can be referenced
by the incoming push. In that case, the object would be deleted because
there is no way to know it is no longer unreferenced. This will leave
the repository in an inconsistent state and most of the operations fail
with a missing tree/object error.

Given the incoming push change the last modified date for the now
referenced object, verify this one is still a candidate to delete
before actually performing the delete operation.

Change-Id: Iadcb29b8eb24b0cb4bb9335b670443c138a60787
Signed-off-by: Hector Oswaldo Caballero <hector.caballero@ericsson.com>