graft: fix --base value to be saved in state file
'True' just works because it is treated as an integer revision '1' and
only the truthiness of the basectx is important. If multiple source revisions
were supported with --base, the resumed graft operation would go wrong.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed