graft: always allow hg graft --base . (issue6248) `hg graft --base . -r abc` is rejected before this change with a "nothing to merge" error, if `abc` does not descend from `.`. This looks like an artifact of the implementation rather than intended behavior. It makes perfect sense to apply the diff between `.` and `abc` to the working copy (i.e. degenerate into `hg revert`), regardless of what `abc` is. Differential Revision: https://phab.mercurial-scm.org/D8127