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