abort: added support for graft
This adds support of `graft` to `hg abort` plan.
The patch creates a seperate function `cmdutil.hgabortgraft`
so that abort logic for graft can be called independently.
This logic is registered to the statedetection API as `abortfunc`.
Results are shown as tests.
Differential Revision: https://phab.mercurial-scm.org/D6567