rebase: clean up rebasestate from active transaction
Previously, rebase assumes the following pattern:
rebase:
with transaction as tr: # top-level
...
tr.__close__ writes rebasestate
unlink('rebasestate')
However it's possible that "rebase" was called inside a transaction:
with transaction as tr1:
rebase:
with transaction as tr2: # not top-level
...
tr2.__close__ does not write rebasestate
unlink('rebasestate')
tr1.__close__ writes rebasestate
That leaves a rebasestate on disk incorrectly.
This patch adds "removefilegenerator" to notify transaction code that the
state file is no longer needed therefore fixes the issue.