graft: never set both parents equal in the dirstate (issue6098) merge.graft() can set both parents equal in the dirstate when keepparent=True. We don't seem to set that in core, but the evolve extension does use it. So I couldn't figure out a way to add a test for this patch in core. Differential Revision: https://phab.mercurial-scm.org/D7549

graft: extract `repo['.']` to local variable It's used in two places and I'm about to use it more. Differential Revision: https://phab.mercurial-scm.org/D7548

graft: rename `pctx` argument to `base` since that's what it is The new name better matches the docstring. It also frees up `pctx` to be used for something else (next patch). Differential Revision: https://phab.mercurial-scm.org/D7547