To avoid duplication of code due to `cmdutil.continuegraft()`; `graft()`
is split into `cmdutil.finishgraft()` which deals with the execution of graft
once `revs` are generated and `cmdutil.continuegraftstate()` which updates opts
from `graftstate` file when graft is continued from an interrupted state.
Further tests for issue1175 are updated to show the behaviour of `hg continue`.