Implemented, but untested, global-removes

This implementation is in theory correct, but of course, being untested,
who knows?

Though this does come with remounting added to all of the directory
tests. This effectively tests that all of the directory creation tests
we have so far maintain grm=0 after each unmount-mount cycle. Which is
valuable.

scripts/dbgrbyd.py

@@ -924,7 +924,8 @@ def main(disk, blocks=None, *,
 
         # compare with sequence arithmetic
         if trunk_ and (
-                not ((rev - revs[i]) & 0x80000000)
+                not trunks_[i]
+                or not ((rev - revs[i]) & 0x80000000)
                 or (rev == revs[i] and trunk_ > trunks_[i])):
             i = i_