Barriers cannot be active during sweep, even in generational mode.
(Although gen. mode is not incremental, it can hit a barrier when
deleting a thread and closing its upvalues.) The colors of objects are
being changed during sweep and, therefore, cannot be trusted.
From the point of view of 'git', all names are relative to the root
directory of the project. So, file names in '$Id:' also should be
relative to that directory: the proper name for test file 'all.lua'
is 'testes/all.lua'.
During generational collection, a userdatum must become gray and
go to a gray list after being traversed (like tables), so that
'correctgraylist' can handle it to its next stage.
This commit also added minimum tests for the generational collector,
including one that would detect this bug.
